Monster cookie bars- soft and chewy peanut butter oatmeal cookie bars loaded with colorful M&Ms and chocolate chips! Inspired by monster cookies, these quick and easy bakery-style dessert bars are made in one bowl in less than 1 hour.
As much as I love M&M cookies and iced oatmeal cookies, I always found the monster cookie bars I made working as a pastry chef were overly sweet, dry, and cakey instead of chewy. I have spent hours testing these monster bars and I've finally perfected them!
The best part of this recipe? No dough chilling or cookie dough scooping- just press the dough into a baking pan, bake, slice, and serve! Try our sugar cookie bars next!
Why We Love Monster Cookie Bars
- Oatmeal monster cookies in bar form!
- Thick and super soft-baked with a chewy texture and crispy edges.
- Sweet, salty, and loaded with gooey chocolate, M&Ms, oats, and peanut butter.
- Quicker to make than my other cookie recipes- no dough chilling or cookie scoop is required.
- Customizable for any occasion.
- Just one bowl and a handful of simple ingredients!
Ingredient Notes and Substitutions
Chocolate chips: Dark chocolate chips or semi-sweet chocolate chips are best for these bars. If you love extra sweet cookie bars, try butterscotch chips, white chocolate chips, or milk chocolate chips. For extra peanut butter flavor, peanut butter chips or Reese's pieces are incredible!!
M&Ms: M&M candies are a must in monster cookie bars! I like using a mixture of mini M&Ms and regular-sized M&Ms for a texture variety. Swap out the multi-color candies for red and green for Christmas or shades of pink for Valentine's Day. Use any extras to make M&M brownies!
Peanut butter: Like my Reese's cake and peanut butter cookies, natural creamy peanut butter is best for this recipe. Other varieties have added sugars, oil, and salt which can affect the overall flavor and texture of the monster cookie bars.
Brown sugar: Light brown sugar makes chewy monster bars with just the right amount of sweetness. Dark brown sugar can be used in substitution.
Find the entire ingredient measurements and instructions in the full recipe card below!
How to Make Monster Bars
Line a 9x13 baking pan with parchment paper. If you don't have any parchment, liberally grease the sides and bottom of the pan with nonstick cooking spray or butter. If you are a visual person, watch the video below on how to make monster cookie bars!
Step 1: Cream the butter, sugar, and peanut butter. In a large mixing bowl with a hand mixer or stand mixer, cream the peanut butter, softened butter, granulated sugar, and brown sugar until light and fluffy.
Step 2: Mix in the wet ingredients. Mix in the egg, egg yolk, and vanilla extract until combined. Scrape down the bowl if necessary.
Step 3: Mix in the dry ingredients. Use a rubber spatula to fold in the flour, baking soda, salt, M&Ms, and chocolate chips until just combined.
Step 4: Bake! Evenly press the monster cookie dough into the prepared baking pan. Sprinkle more M&Ms and chocolate chips over the top of the dough.
Bake until the top and edges are golden brown. Let the bars cool completely on a wire rack, then cut them into 24 squares.
Expert Baking Tips
Use a kitchen scale. Baking with a scale is much more accurate than cup measurements. To convert this recipe, click the "metric" button next to the ingredients title on the recipe card. If you do not have a scale, use a spoon to fluff the flour first, then spoon it into your measuring cup.
Decorate for the holidays! Press cute eyeball candies on top for Halloween treats (or fold in leftover Halloween candy) and switch up the M&M colors for your favorite sports team, birthday party, or holiday.
Use room temperature ingredients for best results. Ingredients that are cold will not combine properly and can result in uneven texture.
Don't overbake them. If you want nice gooey and chewy monster cookie bars, bake them just until the top is golden brown and puffy. A toothpick inserted into the center should come out slightly moist but not covered in thick wet dough.
You can substitute the peanut butter with almond butter if desired!
Store the baked cookie bars in an airtight container at room temperature. They will stay stoft and chewy for 2-3 days!
Using too much flour is usuing the culprit, which is why I highly recommend using a kitchen scale to weigh your ingredients!
Yes! Freeze the bars cut or as a whole in a freezer bag or airtight container for up to one month.
Yes! Cut the recipe in half and bake it in a square 8x8 baking pan for about 20-25 minutes.
More Dessert Bars You'll Love
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Monster Cookie Bars
- 14 tablespoon unsalted butter room temperature
- ½ cup creamy peanut butter
- ½ cup light brown sugar packed
- ½ cup dark brown sugar packed
- ½ 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 ¾ cup rolled oats (old-fashioned oats)
- ½ cup semi-sweet chocolate chips
- ½ cup chopped chocolate bar
- ⅓ cup mini M&Ms
- ⅓ cup regular M&Ms
- 1 teaspoon baking soda
- 1 teaspoon fine sea salt
- Line a 9x13 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.
- In a mixing bowl, cream the butter, peanut butter, brown sugar, and granulated sugar until light and fluffy.14 tablespoon unsalted butter, ½ cup creamy peanut butter, ½ cup light brown sugar, ½ cup dark brown sugar, ½ cup granulated sugar
- Mix in the eggs, egg yolks, and vanilla.2 large eggs, 2 large egg yolks, 1 tablespoon vanilla extract
- 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 all-purpose flour, 1 ¾ cup rolled oats (old-fashioned oats), ½ cup semi-sweet chocolate chips, ½ cup chopped chocolate bar, ⅓ cup mini M&Ms, 1 teaspoon baking soda, 1 teaspoon fine sea salt, ⅓ cup regular M&Ms
- 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!
The calorie information provided for the recipe is an estimate. The accuracy of the calories listed is not guaranteed.