A giant monster cookie that's so thick, soft-baked, chewy, and loaded with all the monster cookie favorites. Oats, chocolate, peanut butter, and m&m's. This giant monster cookie makes for a great first day of school tradition too. Kids will love it as a fun after school snack to celebrate their first day. 

Giant Monster Cookie

One giant monster cookie loaded with peanut butter, oats, chocolate, and m&m's. This is a fun way to celebrate the first day of school. 

Course Dessert
Cuisine American
Prep Time 10 minutes
Cook Time 18 minutes
Total Time 28 minutes
Author Jessica - Together as Family


Giant Monster Cookie

  • 1/2 cup (1 stick) butter, softened
  • 1/2 cup creamy peanut butter
  • 1 cup light brown sugar
  • 1 large egg
  • 2 teaspoons vanilla extract
  • 3/4 cup all-purpose flour
  • 1 teaspoon baking soda
  • 1 teaspoon cornstarch
  • 1/4 teaspoon salt
  • 1 1/2 cups quick oats
  • 1 cup mini chocolate chips
  • 1 cup mini m&m's

Chocolate Frosting (optional)

  • 1/2 cup (1 stick) butter, softened
  • 1/4 cup cocoa powder
  • 1 1/2 cups powdered sugar
  • 2-3 tablespoons heavy cream


  1. Preheat oven to 350 degrees. Spray a 11" tart pan with cooking spray.
  2. In a large bowl (or bowl of a stand mixer) cream together the butter, brown sugar, and peanut butter until light and fluffy, about 3-4 minutes. Mix in the egg and vanilla, scraping down the sides of the bowl as needed.

  3. In a separate bowl, add the flour, baking soda, cornstarch, and salt and whisk to combine. Add the flour mixture to the wet ingredients and mix until just combined.

  4. Add the oats, chocolate chips, and M&M candies. Mix on low speed just until dough is combined.
  5. Spread dough into the prepared tart pan and press evenly with your fingers. Sprinkle a few extra chocolate chips and m&m's on top and gently press down.
  6. Cook for 16-20 minutes or until lightly golden brown. It will cook more as it cools so you don't want to overcook it. Mine is done at 16 minutes. Remove from oven and let cool completely on a cooling rack.

  7. If wanted pipe on some chocolate frosting: Whip butter in a bowl with a handheld blender until light and fluffy. Add cocoa powder, powdered sugar, and heavy cream and mix. Use more or less powdered sugar and heavy cream depending on the desired consistency. Pipe onto edges of cooled cookie and decorate with additional m&m's.

Recipe Notes

I found my tart pan on amazon. 


You will find the mini m&m's in the candy aisle and also sometimes with the chocolate chips. 


I have made this with and without the frosting. Both ways are yummy. With the frosting it's sweet, so keep that in mind if you are sensitive to sweetness.