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 spoonful's of the monster cookie dough over the brownie batter OR you can use your hands, grab some cookie dough and flatten it out (as best you can) and lay it on top of the brownie batter. This is how I do it and it gets an even layer on top of the brownies. Either way works just fine!
Bake for 25-35 minutes or until done in the center. Use a toothpick to make sure it's not wet in the center before pulling them out. Mine usually take around 30 minutes to cook to my liking.
Let cool in the pan for 30 minutes to 1 hour and then cut into squares. Store any leftover covered at room temperature.
Make sure that your brownie mix is for a 9x13 baking pan. I think on some boxes it's called 'family size' or just follow the recommended ounces size in the recipe.
Linking this recipe at the weekend potluck