BEST Monster Cookie Bars

Name: Monster Cookie Bars
Prep time: 15 min
Cook time: 25 min
Total time: 40 min
Ingredients: 14 tbsp 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 tbsp 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

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

Ingredients for monster cookie bars in bowls with labels.

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.

Oats: For texture and structure, use rolled oats (also known as old-fashioned oats). For a less "oatmeal" texture, use quick oats!

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!

A mixing bowl with the creamed butter, sugar, and peanut butter.

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.

A mixing bowl with the creamed wet ingredients.

Step 2: Mix in the wet ingredients. Mix in the egg, egg yolk, and vanilla extract until combined. Scrape down the bowl if necessary.

A mixing bowl with the oatmeal cookie dough with mm and chocolate chips on top.

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.

A baking pan with the peanut butter cookie dough pressed into the pan with extra candies and chocolate chips on top.

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.

FAQs

Can I make them without peanut butter?

You can substitute the peanut butter with almond butter if desired!

How should monster cookie bars be stored?

Store the baked cookie bars in an airtight container at room temperature. They will stay stoft and chewy for 2-3 days!

Why are my cookie bars dry and crumbly?

Using too much flour is usuing the culprit, which is why I highly recommend using a kitchen scale to weigh your ingredients!

Can you freeze monster cookie bars?

Yes! Freeze the bars cut or as a whole in a freezer bag or airtight container for up to one month.

I want to make a small batch instead, can I use a different sized pan?

Yes! Cut the recipe in half and bake it in a square 8x8 baking pan for about 20-25 minutes.