This is the ULTIMATE cookie bar recipe! Monster cookie bars combine all of your favorite cookies (chocolate chip, m&m, peanut butter and oatmeal) to create the best dessert bars. They are thick, chewy and full of flavor. You can adapt them to any holiday by switching out the M&M colors!

monster cookie bar with m&ms on parchment paper.


Quick and easy dessert bars are my favorite for entertaining. These monster cookie bars are always a crowd pleaser because they are a combination of all the best cookies (a Frankenstein of cookie bars if you will). You can make a big batch, then just slice and serve to your guests!

Why this recipe works

It is baked in a 9×13 pan for large batch dessert bars—perfect for entertaining. They are quicker to make than cookies because you don’t have to scoop and roll out the dough. Just bake, cut and serve! You can use different color M&M’s depending on what holiday is next.

monster cookie bars ingredients in bowls labeled with text.

Ingredient notes

  • Oats – rolled oats (also known as old-fashioned oats) are best for this recipe. They provide the best chewy texture. In a pinch, you can substitute with quick oats. I do not recommend steel-cut oats.
  • Peanut butter – No-stir creamy peanut butter makes thick and flavorful bars (I prefer Skippy all natural peanut butteR). Do not use organic peanut butter with oil sitting on top. It will make a gooey mess!
  • Sugars – The combination of brown sugar and granulated sugar makes for sweet, yet chewy bars. You can use light or dark brown sugar.

Recipe variations

My favorite thing about this dessert bar is that you personalize the recipe for each holiday. For Halloween cookie bars I like using orange and purple M&M’s with candy eyes pressed on top of the bars. Around Christmas, I always opt for red and green M&M’s.

If you want triple peanut butter cookie bars, substitute chocolate chips with peanut butter chips and swap out the m&m’s for Reese’s Pieces. If you do not want to use peanut butter, you can try no-stir almond butter, but the texture will be slightly different.

photo collage demonstrating how to make monster cookie bars in a mixing bowl and 9x13 pan.
  1. Whisk together the dry ingredients—flour, oats, baking soda and salt. Set aside.
  2. Mix the wet ingredients—beat the brown sugar, sugar, peanut butter and butter until light and fluffy. Add the egg, egg yolks and vanilla.
  3. Combine the wet ingredients and dry ingredients. Mix just until combined. Stir in the M&M’s by hand.
  4. Bake the bars Transfer dough to a 9×13 pan and bake at 350 F for 25 to 30 minutes or until the edges are set and center is soft. Cool completely before cutting and serving.

Expert tips

Use room temperature ingredients to create soft and chewy bars. Room temperature ingredients guarantee a uniform dough with good consistency. When stirring in the dry ingredients, mix just until combined. If you over-mix the dough, the bars will be dense and firm.

Don’t over-bake the bars. For gooey bars, you want the center slightly under baked (the bars will continue baking as they cool). You know the bars are done when the edges are set and turning golden in color.

monster cookie bars with m&ms cut into squares.

Frequently Asked Questions

Why are my cookie bars dry?

Either there is too much flour or they were over baked. If you don’t have a kitchen scale, fluff up your flour with a fork and spoon it into a measuring cup. Level if off with a knife (don’t pack it down). Use an oven thermometer to make sure your oven temperature is accurate.

Can I make monster cookie bars without peanut butter?

Yes, but the texture will be slightly different. I recommend no-stir almond butter. I have not tried this recipe with sunflower butter.

Can you freeze these bars?

Yes, once the bars have cooled completely, store them in an airtight container or freezer bag in the freezer for up to 3 months. For best results, wrap the bars in plastic wrap to prevent freezer burn.

Did you love this recipe? Please leave a 5-star 🌟🌟🌟🌟🌟 rating in the recipe card below and if you REALLY loved it, consider leaving a comment further down the page.

square monster cookie bar on top of more bars with m&ms.

Get the Recipe: Monster Cookie Bars

Monster cookie bars combine all of your favorite cookies (chocolate chip, m&m, peanut butter and oatmeal) to create the best dessert bars.
5 (12 ratings)


  • 1 ½ cups (280 g) all-purpose flour
  • 1 ½ cups old fashioned oats
  • 1 teaspoon baking soda
  • ½ teaspoon 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 pure vanilla extract
  • 1 cup (170 g) semisweet chocolate chips
  • 1 cup M&M’s chocolate candies, plus more for sprinkling on top


  • Preheat oven to 350° F. Line an 9×13 baking pan with parchment paper and set aside.
  • In a medium bowl, whisk together flour, oats, baking soda and salt. Set aside.
  • In the bowl of a stand mixer (or using a hand mixer), beat butter, brown sugar and sugar on medium-high speed until light in color, about 3 minutes. Add peanut butter, egg, egg yolk and vanilla. Mix until incorporated. Add flour mixture to wet ingredients and mix just until combined. Stir in chocolate chips and M&M’s by hand.
  • Transfer dough to prepared baking dish and press down evenly using a spatula or your hands. Add some extra M&M’s on top and press down slightly.
  • Bake for 25 to 30 minutes or until the edges are set and the center is soft. Allow bars to cool in the pan for 10 minutes before lifting out of the pan and letting them cool completely. Slice and serve.


Oats: Rolled oats work best for a chewy texture, but quick oats may be used as a substitution.
Store bars in an airtight container at room temperature for up to 5 days.
Freeze bars in an airtight container or freezer bag for up to 3 months. For best results, wrap bars in plastic wrap.
Serving: 1bar, Calories: 249kcal, Carbohydrates: 31g, Protein: 4g, Fat: 12g, Saturated Fat: 6g, Polyunsaturated Fat: 1g, Monounsaturated Fat: 3g, Trans Fat: 0.2g, Cholesterol: 27mg, Sodium: 130mg, Potassium: 114mg, Fiber: 2g, Sugar: 20g, Vitamin A: 162IU, Vitamin C: 0.1mg, Calcium: 30mg, Iron: 1mg

Share This: