Heat oven to 350 degrees. Prepare a 9x13 baking pan and spray with cooking spray, line with parchment paper, or line with tin foil (be sure to spray the tin foil with spray if you use it).

Prepare brownies according to the direction on the box. Pour brownie batter into the baking pan and set aside.

In a mixing bowl, add the peanut butter and softened butter. Stir with a fork until combined. Add brown sugar, sugar, egg, and vanilla extract. Stir with the fork until combined and mixed together.

Add the flour, baking soda, and salt. Stir just until combined. Add the quick oats, chocolate chips, and M&M's. Stir again until combined.

Drop spoonfuls of the monster cookie dough over the brownie batter or use your hands to grab some cookie dough, flatten it out, and lay it on top of the brownie batter.

Bake for 25-35 minutes or until done in the center. Use a toothpick to check for doneness.

Let cool in the pan for 30 minutes to 1 hour and then cut into squares. Store any leftover covered at room temperature.