This is the ultimate cookie bar recipe. Monster cookie bars combine the best elements of chocolate chip, M&M, peanut butter and oatmeal cookies into thick, chewy dessert bars that are bursting with flavor. They’re easy to adapt for holidays by swapping M&M colors.
Dessert bars are my go-to for entertaining because they’re quick to make and easy to serve. Monster cookie bars are always a crowd-pleaser — think of them as a mash-up of your favorite cookie flavors baked in one pan. Make a large batch, slice and serve for a fuss-free dessert.
Table of Contents
Why this recipe works
Baked in a 9×13 pan, these bars yield a large batch ideal for gatherings. They save time compared to baking individual cookies since you press the dough into a pan, bake, cool and slice. The combination of oats, peanut butter and chocolate creates a chewy, flavorful bar, and M&M colors make them festive for any occasion.
Ingredient notes
- Oats – Rolled (old-fashioned) oats give the best chewy texture. Quick oats work in a pinch. Avoid steel-cut oats.
- Peanut butter – Use a no-stir creamy peanut butter for consistent texture. Avoid natural peanut butter with separated oil, which can make the bars greasy.
- Sugars – A mix of brown and granulated sugar balances sweetness and chewiness. Light or dark brown sugar both work.
Recipe variations
One of the best parts of this recipe is how easy it is to customize. For Halloween, use orange and purple M&M’s and press candy eyes on top. For the holidays, switch to red and green M&M’s. To emphasize peanut butter, replace chocolate chips with peanut butter chips and swap M&M’s for Reese’s Pieces. If you prefer not to use peanut butter, try a no-stir almond butter — the texture will be slightly different.
If you’d rather bake cookies instead of bars, you can use the same ingredients to make monster cookies by portioning the dough into individual cookies and adjusting baking time accordingly.
How to make monster cookie bars
- Preheat and prepare — Preheat oven to 350°F and line a 9×13 pan with parchment paper.
- Whisk dry ingredients — Combine flour, oats, baking soda and salt in a medium bowl and set aside.
- Cream wet ingredients — In a stand mixer or with a hand mixer, beat butter, brown sugar and granulated sugar until light, about 3 minutes. Add peanut butter, egg, egg yolk and vanilla and mix until incorporated.
- Combine and mix-ins — Add the dry mixture to the wet ingredients and mix just until combined. Fold in chocolate chips and M&M’s by hand.
- Press and bake — Press the dough evenly into the prepared pan. Sprinkle extra M&M’s on top and press lightly. Bake 25–30 minutes, until edges are set and the center is still slightly soft. Cool completely before slicing.
Expert tips
Room temperature ingredients help create a uniform, soft dough. When adding the dry ingredients, mix only until combined to avoid dense bars.
Watch baking time — For gooey, chewy bars, remove them when the edges are set but the center remains slightly soft. They will continue to firm up as they cool.
Frequently Asked Questions
Dry bars usually mean too much flour or overbaking. If you measure by volume, fluff the flour, spoon it into the cup and level it off instead of packing. An oven thermometer helps ensure your oven runs at the correct temperature.
Yes. Substitute a no-stir almond butter if desired; texture will change slightly. Sunflower butter has not been tested in this recipe.
Yes. Once fully cooled, wrap bars in plastic and store in an airtight container or freezer bag for up to 3 months to prevent freezer burn.
Related recipes
If you love dessert bars, try these other cookie-inspired recipes.
Chocolate Chip Cookie Bars
Reese’s Pieces Peanut Butter Blondies
Chewy Snickerdoodle Bars
Sugar Cookie Bars
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Monster Cookie Bars
Equipment
- 1 Kitchen scale
- 1 Stand mixer or hand mixer
- 1 9×13 metal baking pan
Ingredients
- 1 ½ cups (180 g) all-purpose flour
- 1 ½ cups (127 g) old fashioned oats
- 1 teaspoon (6 g) baking soda
- ½ teaspoon (2 g) salt
- ½ cup (113 g) unsalted butter, room temperature
- ½ cup (113 g) no-stir creamy peanut butter
- ¾ cup (150 g) packed light brown sugar
- ½ cup (100 g) granulated sugar
- 1 egg, room temperature
- 1 egg yolk, room temperature
- 2 teaspoons (10 ml) pure vanilla extract
- 1 cup (170 g) semisweet chocolate chips
- 1 cup (211 g) M&M’s, plus more for topping
Instructions
- Preheat oven to 350°F. Line a 9×13 baking pan with parchment paper.
- Whisk together flour, oats, baking soda and salt in a medium bowl. Set aside.
- In a stand mixer or with a hand mixer, beat butter, brown sugar and granulated sugar until light, about 3 minutes. Add peanut butter, egg, egg yolk and vanilla; mix until combined. Add the flour mixture and mix just until incorporated. Fold in chocolate chips and M&M’s by hand.
- Press dough evenly into the prepared pan. Sprinkle extra M&M’s on top and press lightly.
- Bake 25–30 minutes, until edges are set and the center is still slightly soft. Cool in the pan for 10 minutes, then lift out and cool completely before slicing and serving.
Notes
Oats: Rolled oats give the best chew; quick oats are an acceptable substitute.
Storage: Store at room temperature in an airtight container up to 5 days.
Freezing: Wrap in plastic and freeze in an airtight container up to 3 months.
, Calories: 249 kcal