Ultimate Chewy Monster Cookie Bars: Easy One-Bowl Recipe with M&Ms & Peanut Butter
Get ready to indulge in the ultimate treat! Inspired by classic monster cookies, these incredible monster cookie bars are a baker’s dream come true. Imagine thick, chewy peanut butter oatmeal bars generously packed with vibrant M&M candies and luscious chocolate chips. The best part? This mouth-watering dessert is made in just one bowl and bakes up beautifully in under an hour, making it perfect for last-minute cravings or spontaneous gatherings.
Monster cookies have always been a consistent top seller at every bakery I’ve had the pleasure of working at. This enduring popularity is exactly what inspired me to create these incredibly fun, irresistibly easy, and deeply satisfying monster cookie bars! Unlike traditional monster cookies that often require dough chilling and meticulous scooping—like my M&M cookies—this recipe simplifies the process entirely. There’s no need to wait around; just press the rich, vibrant dough into a pan and pop it into the oven.
These monster cookie bars are wonderfully thick, delightfully chewy, and generously loaded with pockets of gooey melted chocolate, an abundance of colorful M&Ms (much like my popular monster cookie cupcakes and M&M brownies), hearty oats, and creamy peanut butter. Their customizable nature makes them absolutely perfect for any occasion, whether you’re celebrating a holiday, hosting a party, or simply treating yourself to a delightful homemade dessert.
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Best Mix-Ins for Your Monster Cookie Bars
The beauty of these chewy peanut butter oatmeal bars lies in their incredible versatility. While the core recipe is a masterpiece, adding a variety of mix-ins allows you to truly make them your own. Here are some of the best additions to elevate your monster cookie bars:
M&Ms: These colorful candy-coated chocolates are a signature element of monster cookies. You can use classic regular M&Ms, adorable mini M&Ms, or even a fun combination of both for an extra pop of color and texture. Feel free to use holiday-themed M&Ms to match any celebration!
Chocolate Chips: A non-negotiable for that irresistible gooey, melty texture in every bite. Don’t limit yourself to just one kind! Experiment with a mix of semi-sweet, creamy milk chocolate, rich dark chocolate, or sweet white chocolate chips to create complex flavor profiles.
Peanut Butter Chips: For an intense burst of peanut butter flavor, you can replace some of the regular chocolate chips with peanut butter chips. Alternatively, swirl in some Reese’s pieces for an even more pronounced peanut butter and chocolate experience that truly lives up to the “monster” name.
Pretzels: Craving a delightful textural contrast? Crushed pretzels are an excellent addition. They introduce a fantastic sweet and salty crunch that balances the richness of the cookie bars beautifully.
Other Nuts: Enhance the nutty flavor by adding chopped walnuts, pecans, or even roasted peanuts. Just be mindful of any nut allergies if you’re serving a crowd.
Butterscotch Chips or Toffee Bits: For a different kind of sweet, caramel-like flavor, butterscotch chips or crunchy toffee bits can add another layer of deliciousness.
Why You’ll Fall in Love with These Easy Monster Cookie Bars
There are countless reasons why these easy monster cookie bars will quickly become a staple in your baking repertoire. Beyond their undeniable deliciousness, they offer unparalleled convenience and versatility:
- Effortless Preparation: Forget complicated recipes and multiple bowls. This streamlined recipe comes together in just one bowl, significantly cutting down on cleanup time. It’s perfect for busy bakers or anyone new to the kitchen.
- No Chilling Required: Unlike many cookie recipes that demand hours of chilling, this dough is ready to bake immediately. Simply mix, press, and bake!
- Phenomenal Flavor Profile: These bars strike the perfect balance between sweet and salty, with the comforting richness of peanut butter, the chewy texture of oats, and the delightful burst of M&M’s and chocolate chips in every bite.
- Irresistible Texture: Expect a truly satisfying texture – soft, incredibly chewy, and wonderfully gooey in the center, with slightly crisp edges.
- Crowd-Pleasing Perfection: Baking in a 9×13 inch pan means you get perfectly portioned bars that are easy to slice and serve. They are ideal for potlucks, parties, bake sales, or family gatherings. No individual cookie scooping required!
- Endlessly Customizable: With a base this good, the possibilities for customization are truly endless. Adapt them for holidays, team colors, or simply your favorite candy combinations.
Essential Ingredients for Irresistible Monster Cookie Bars
Creating bakery-quality chewy monster cookie bars starts with understanding the role of each ingredient. Here’s a closer look at what you’ll need to achieve optimal flavor and texture:
- Unsalted Butter (Room Temperature): Using unsalted butter gives you control over the salt content. Ensuring it’s at room temperature is crucial for proper creaming with the sugars and peanut butter, leading to a light, fluffy, and well-aerated dough.
- Creamy Peanut Butter (No-Stir): For the best results, opt for creamy, “no-stir” peanut butter. These varieties typically have a more consistent texture and less oil separation, which contributes to a smoother dough and better final texture in your peanut butter oatmeal bars. Avoid natural peanut butter that requires stirring, as its oil content can vary and affect the recipe.
- Sugars (Granulated, Light Brown, Dark Brown): This recipe uses a trifecta of sugars. Granulated sugar provides crispness and sweetness. Light brown sugar adds moisture and a subtle caramel note, contributing to chewiness. Dark brown sugar further enhances moisture, chewiness, and depth of flavor with its higher molasses content.
- Large Eggs & Egg Yolks (Room Temperature): Eggs act as a binder and provide richness. The addition of extra egg yolks contributes significantly to the chewy, fudgy texture that makes these bars so decadent. Room temperature eggs emulsify better with the butter and sugar, resulting in a smoother batter.
- Vanilla Extract: A crucial flavor enhancer. Use good quality pure vanilla extract for the best aroma and taste.
- All-Purpose Flour: The structural base of your cookie bars. Proper measurement is key! See our “Top Tips” section for accurate flour measurement.
- Rolled Oats (Old-Fashioned Oats): These are essential for the classic “oatmeal” texture in your peanut butter oatmeal bars. Rolled oats retain their shape and provide a satisfying chew. Quick oats can be used in a pinch, but they will result in a less defined oatmeal texture.
- Baking Soda: This leavening agent reacts with the acidic brown sugar to help the bars rise and spread slightly, contributing to their soft texture.
- Fine Sea Salt: Balances the sweetness and enhances all the other flavors, particularly the peanut butter and chocolate.
- M&Ms & Chocolate Chips: The star mix-ins! Use your favorite combination to make these bars truly shine.
Top Baking Tips for Perfect Monster Cookie Bars
Achieving bakery-quality results with your monster cookie bars is easier than you think, especially when you follow these key tips:
1. Don’t overmix the cookie dough. Once you’ve added the dry ingredients (flour, oats, baking soda, salt), mix just until they are barely combined and no streaks of flour remain. Overmixing develops the gluten in the flour, which can lead to tough, dry cookie bars instead of the desired soft and chewy texture.
2. Bake until golden brown. For that irresistible gooey and chewy center, bake your peanut butter oatmeal bars until the top appears set, slightly puffy, and a beautiful golden brown color, especially around the edges. A quick test with a toothpick inserted into the center should come out slightly moist with a few crumbs, but not covered in thick, wet dough. An overly clean toothpick means they might be overbaked.
3. Cool completely before cutting. This step is non-negotiable for perfectly structured bars. Allow the monster cookie bars to cool fully in the pan on a wire rack before attempting to cut them. As they cool, the molten chocolate and gooey center will set, preventing the bars from falling apart and ensuring clean, tidy slices. Patience is truly a virtue here!
4. Room Temperature Ingredients are Key: This tip is mentioned throughout baking, and for good reason! Softened butter and room temperature eggs cream together smoothly, creating a uniform emulsion that traps air. This leads to a lighter, more consistent dough and a superior texture in the final baked goods.
5. Prepare Your Pan Properly: Lining your 9×13 inch pan with parchment paper, with an overhang on the sides, is a game-changer. It not only prevents sticking but also allows you to easily lift the entire slab of cooled cookie bars out of the pan for effortless cutting.
Endless Ways to Customize Your Monster Cookie Bars
One of the most delightful aspects of these M&M chocolate chip bars is how easily they can be adapted to suit any theme, holiday, or personal preference. Let your creativity shine!
Spooky Halloween Treats: Make your monster cookie bars truly monstrous! Press edible candy eyeballs onto the warm bars immediately after baking. You can also use seasonal orange and black M&Ms or mix in chopped leftover Halloween candy for a fun, ghoulish twist.
Game Day Colors: Show your team spirit! Customize your peanut butter oatmeal bars by using M&Ms in your favorite sports team’s colors. This is a fantastic way to celebrate game day with a delicious and festive treat—go sports!
Festive for Christmas: Embrace the holiday season with vibrant red and green M&Ms. For an extra touch of elegance, drizzle the cooled bars with melted white chocolate and sprinkle with festive edible glitter or crushed candy canes.
Valentine’s Day Shades: Spread the love with pink and red M&Ms and an assortment of adorable heart sprinkles. These sweet bars are sure to capture hearts on Valentine’s Day.
Patriotic Celebrations: For Fourth of July or Memorial Day, use red, white, and blue M&Ms for a festive and delicious dessert that’s perfect for barbecues and picnics.
Birthday Fun: Choose M&Ms in the birthday person’s favorite colors and add some celebratory sprinkles for a personalized birthday treat.
Choosing the Right Baking Pan for Success
The type and size of your baking pan play a more critical role than you might think in the success of your chewy monster cookie bars. Using the right pan ensures even baking and the perfect texture.
Choose a light-colored metal pan: When baking cookies or bars, the material and color of your pan significantly impact how heat is conducted. Glass, ceramic, or dark-colored metal pans absorb and retain heat differently, which can lead to uneven baking—often resulting in burnt bottoms and underbaked centers. A light-colored metal pan reflects heat, promoting a more even bake and preventing overly browned or crispy edges, ensuring your peanut butter oatmeal bars are perfectly tender throughout. If you must use a darker pan, consider reducing the oven temperature by 25°F (15°C) and adjusting baking time accordingly.
Opt for a 9×13-inch pan: This specific size is truly ideal for these monster cookie bars. It provides the perfect surface area to achieve just the right thickness for a chewy, satisfying bar without being too thin and crispy, or too thick and doughy in the center. If you use a pan that is too small, your bars will be too thick and require a longer baking time, increasing the risk of overbaking the edges. If your pan is too large, the bars will be thin and might bake too quickly, resulting in a drier texture. Stick to the 9×13 inch pan for optimal results!
Ingredient Notes and Substitutions for Best Results
To ensure your M&M chocolate chip bars turn out perfectly every time, pay close attention to these ingredient details and consider these substitutions:
Peanut butter: For optimal texture and flavor, a natural, creamy, no-stir peanut butter is highly recommended, much like in my delicious nutter butter bars. Other varieties, especially those that require stirring, can have varying amounts of added sugars, oils, and salt, which can significantly alter the overall flavor, sweetness, and the moist, chewy texture of your monster cookie bars. If you need a substitution due to allergies or preference, almond butter is an excellent alternative that will yield similar results in terms of consistency.
Oats: The choice of oats is crucial for the signature texture of these peanut butter oatmeal bars. For the best, most substantial and chewy texture, always use rolled oats (also known as old-fashioned oats). These retain their shape and provide a satisfying chewiness. While you can use quick oats in a pinch, be aware that the bars will have a finer, less distinct oatmeal texture. Quick oats absorb liquids faster, which can sometimes lead to a slightly drier bar if not adjusted.
Gluten-Free Option: To make these monster cookie bars gluten-free, substitute the all-purpose flour with a 1:1 gluten-free baking flour blend that contains xanthan gum. Ensure your rolled oats are certified gluten-free, as oats can sometimes be cross-contaminated during processing. Always check all other ingredient labels for hidden gluten.
Nut-Free Option: If you need to make these bars nut-free, substitute the peanut butter with a seed butter like sunflower seed butter (SunButter). Be sure to double-check that your M&Ms and chocolate chips are also certified nut-free, as cross-contamination can occur in manufacturing.
Step-by-Step Recipe Instructions
Follow these detailed steps to create perfectly baked chewy monster cookie bars that everyone will adore:
Step 1: Prepare your pan and cream the butter, sugar, and peanut butter. Begin by lining a 9×13 inch baking pan with parchment paper, leaving an overhang on the longer sides for easy removal. Alternatively, grease the sides and bottom generously with nonstick spray or butter. Preheat your oven to 350°F (180°C). In a large mixing bowl, using either a hand mixer or a stand mixer fitted with the paddle attachment, cream together the softened unsalted butter, creamy peanut butter, granulated sugar, and both light and dark brown sugars. Beat on medium speed for 2-3 minutes until the mixture is visibly lightened in color and fluffy in texture. This step incorporates air, contributing to the bars’ soft chewiness.
Step 2: Mix in the wet ingredients. Once the butter and sugars are creamed, add the large eggs, extra egg yolks, and vanilla extract to the mixture. Beat on medium-low speed until all ingredients are just combined and fully incorporated. Be sure to scrape down the sides and bottom of the bowl with a spatula as needed to ensure everything is evenly mixed. Overmixing at this stage can also start to develop gluten, so mix only until smooth.
Step 3: Mix in the dry ingredients and mix-ins. In a separate small bowl, whisk together the all-purpose flour, rolled oats, baking soda, and fine sea salt. Gradually add this dry ingredient mixture to the wet ingredients in the large bowl. Using a rubber spatula, gently fold until the flour mixture is *just* incorporated, and no dry streaks remain. Then, fold in the M&Ms (both mini and regular), semi-sweet chocolate chips, and chopped chocolate bar. Mix only until the add-ins are evenly distributed. Remember, avoid overmixing to keep your bars tender and chewy!
Step 4: Press into pan and bake! Once the dough is ready, evenly press it into the prepared 9×13 inch baking pan. You can use your hands (lightly floured if sticky) or the back of a spatula to ensure an even layer. For a beautiful presentation, sprinkle a few extra M&Ms and chocolate chips over the top of the pressed dough before baking. Bake in the preheated oven for 25-35 minutes, or until the top and edges are golden brown and a toothpick inserted into the center comes out with moist crumbs. Be careful not to overbake, as this can lead to dry bars. Once baked, remove the pan from the oven and let it cool completely on a wire rack before cutting into squares. This cooling step is vital for the bars to set and maintain their shape. Enjoy these delightful easy cookie bar recipe treats!
More Top Tips for Bakery-Style Results
Use a kitchen scale. This is hands down my absolute #1 baking tip for consistent, perfect results every time, especially when it comes to flour! Baking with a scale ensures precise measurements, which is critical for the delicate balance of ingredients in chewy cookie bars. If you do not have a scale, remember to use the “spoon and level” method: fluff the flour in its container first, then gently spoon it into your measuring cup until overflowing, and finally, level it off with a straight edge. Never scoop the measuring cup directly into the flour, as this compacts it and can lead to using up to 25% too much flour, resulting in dry, crumbly bars.
Use room temperature ingredients. This tip bears repeating because it truly makes a difference. Ensure your butter and eggs sit out until they reach room temperature. This allows them to cream together much more effectively, forming a smooth, emulsified mixture that traps air. The result? Soft, incredibly chewy, and perfectly textured monster cookie bars every single time.
Don’t Overbake: When in doubt, err on the side of underbaking slightly. The bars will continue to cook and set as they cool in the pan. An underbaked bar can still be wonderfully gooey, but an overbaked bar will be dry and hard.
Press Evenly: When pressing the dough into the pan, ensure it’s spread in an even layer from edge to edge. This promotes uniform baking and consistent thickness in all your M&M chocolate chip bars.
Serving and Pairing Suggestions
These chewy peanut butter oatmeal bars are incredibly versatile and can be enjoyed in many ways. Serve them warm with a scoop of vanilla bean ice cream melting on top for an extra indulgent dessert, or at room temperature for a perfect snack or treat. They pair wonderfully with a tall glass of cold milk, a hot cup of coffee, or even a robust tea. Pack them into lunchboxes, bring them to a potluck, or arrange them on a dessert platter for any celebration. They’re guaranteed to be a hit!
DIY Gift Giving: Monster Cookie Bar Mix in a Jar
Looking for a thoughtful and fun gift? Turn this delicious monster cookie bars recipe into the perfect DIY present by layering the dry ingredients in a beautiful jar! This personalized gift is ideal for holidays, birthdays, teacher appreciation, or just because. Carefully layer the rolled oats, followed by the different sugars, baking soda, and salt. Then add the vibrant M&Ms and chocolate chips on top, creating a visually appealing presentation. Attach a cute label or tag with the remaining wet ingredients needed (butter, peanut butter, eggs, vanilla) and clear baking instructions. It’s a delightful, homemade gesture that anyone who loves to bake (or eat!) will truly appreciate and enjoy.
Storage and Freezing Instructions
Proper storage is key to keeping your easy cookie bar recipe treats fresh and delicious for as long as possible:
Storage: Once completely cooled, leftover monster cookie bars can be stored in an airtight container at room temperature for 2-3 days. To maintain their soft and chewy texture, I do not recommend refrigerating them, as the cold air can dry them out and make them firm. If you live in a very warm or humid climate, you might store them in the fridge, but bring them to room temperature before serving for the best texture.
Freezer: These peanut butter oatmeal bars freeze beautifully! You have a couple of options: you can freeze the entire slab of cooled bars in the pan (if freezer-safe and well-wrapped), or cut them into individual bars first. For individual freezing, wrap each bar tightly in plastic wrap, then place them in a freezer-safe bag or an airtight container. They will stay fresh in the freezer for up to 3 months. When ready to enjoy, simply thaw them at room temperature for an hour or two, or gently warm them in the microwave for a few seconds for that fresh-baked gooeyness.
Frequently Asked Questions (FAQs)
Absolutely! While peanut butter is a signature flavor, you can easily substitute it with almond butter for a slightly different nutty profile, or use a seed butter like sunflower seed butter (SunButter) if you need a nut-free option. Ensure all other ingredients, like M&Ms and chocolate chips, are also certified nut-free if this is for an allergy.
This is an extremely common issue in baking and is almost always due to one of two main reasons: using too much flour or overbaking. Ensure you’re measuring your flour accurately, ideally by weight with a kitchen scale, or using the “spoon and level” method. Also, keep a close eye on your oven and remove the bars as soon as the edges are golden brown and the center is just barely set, as they will continue to cook as they cool.
Yes, you can! If you prefer thicker bars or a smaller batch, you can cut the recipe in half and bake it in an 8×8 inch square baking pan. Adjust the baking time to approximately 20-25 minutes, keeping an eye on it, as smaller pans often require less time. Remember the toothpick test!
Yes, with a few adjustments! Replace the all-purpose flour with an equal amount of a 1:1 gluten-free baking flour blend that contains xanthan gum. Additionally, ensure that the rolled oats you use are certified gluten-free to avoid any cross-contamination. Always check all other ingredient labels for hidden gluten to be completely safe.
Look for golden brown edges and a slightly puffed, set center. The center might still look a little soft, but that’s perfectly normal as the bars will continue to firm up as they cool. Insert a toothpick into the center; it should come out with moist crumbs, not wet batter. If it comes out completely clean, they might be slightly overbaked, but still delicious!
You can use any variety you like! Regular M&Ms provide classic crunch and color, while mini M&Ms offer a more delicate texture and disperse more evenly. Seasonal M&Ms are fantastic for holiday customization. Feel free to mix and match for extra visual appeal and flavor combinations in your M&M chocolate chip bars.
More Delicious Cookie Bar Recipes
If you’re a fan of easy and delicious cookie bars, you’re in luck! Here are some more irresistible recipes to add to your baking list, perfect for any occasion:
- Chocolate Chip Cookie Dough Bars: All the joy of cookie dough in a baked bar, without the raw egg worry!
- Sugar Cookie Bars: Soft, chewy sugar cookies in bar form, perfect for decorating for any holiday.
- Poptart Cookie Bars: A fun, nostalgic twist, combining the flavors of your favorite breakfast pastry with a delicious cookie bar.
- Nutella Cookie Bars: Indulgent, rich, and full of that irresistible hazelnut-chocolate spread.
These monster cookie bars are truly a celebration of flavor, texture, and ease. Whether you’re a seasoned baker or just starting out, this recipe promises delightful results with minimal fuss. Enjoy the chewy goodness, the vibrant M&M’s, and the rich peanut butter flavor in every bite!
If you tried this easy monster cookie bar recipe or any other recipe on my website, please let me know how it went in the comments below; I love hearing from you! Also, please leave a star rating while you’re there! You can also tag me on Instagram or Facebook so I can check it out!
Monster Cookie Bars
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Equipment
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9×13 baking pan
Ingredients
- 14 tablespoons unsalted butter , room temperature
- 1/2 cup creamy peanut butter
- 1/2 cup light brown sugar, packed
- 1/2 cup dark brown sugar, packed
- 1/2 cup granulated sugar
- 2 large eggs, room temperature
- 2 large egg yolks, room temperature
- 1 tablespoon vanilla extract
- 2 cups all-purpose flour, *See notes below for measuring*
- 1 3/4 cup rolled oats (old-fashioned oats)
- 1/2 cup semi-sweet chocolate chips
- 1/2 cup chopped chocolate bar
- 1/3 cup mini M&Ms
- 1/3 cup regular M&Ms
- 1 teaspoon baking soda
- 1 teaspoon fine sea salt
Instructions
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Line a 9×13 baking pan with parchment paper or grease the sides and bottom with nonstick spray or butter. Then preheat the oven to 350 F/180 C.
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In a mixing bowl, cream the butter, peanut butter, brown sugar, and granulated sugar until light and fluffy.14 tablespoons (196 g) unsalted butter, 1/2 cup (132 g) creamy peanut butter, 1/2 cup (100 g) light brown sugar, 1/2 cup (100 g) dark brown sugar, 1/2 cup (100 g) granulated sugar
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Mix in the eggs, egg yolks, and vanilla.2 (100 g) large eggs, 2 (36 g) large egg yolks, 1 tablespoon vanilla extract
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Fold in the flour, oats, baking soda, salt, chocolate chips, chopped chocolate, and M&Ms until just combined. Do not overmix! Evenly spread the cookie dough into the prepared baking pan. Sprinkle the top with extra chocolate chips, chopped chocolate, and M&Ms if desired.2 cups (260 g) all-purpose flour, 1 3/4 cup (180 g) rolled oats (old-fashioned oats), 1/2 cup (85 g) semi-sweet chocolate chips, 1/2 cup (78 g) chopped chocolate bar, 1/3 cup (75 g) mini M&Ms, 1 teaspoon baking soda, 1 teaspoon fine sea salt, 1/3 cup (78 g) regular M&Ms
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Bake in the preheated oven for 25-35 minutes or until the top is golden brown and a toothpick inserted into the center comes out just barely moist. Let them cool completely on a wire rack before cutting then enjoy!
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Notes
The calorie information provided for the recipe is an estimate. The accuracy of the calories listed is not guaranteed.