Heat oven to 350 degrees Fahrenheit. Prepare a 9x13 baking pan with cooking spray, parchment paper, or tin foil (spray the tin foil if used).

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 until combined. Add brown sugar, granulated sugar, egg, and vanilla extract. Stir until mixed.

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

Drop spoonfuls of monster cookie dough over the brownie batter, flattening it out for an even layer if preferred.

Bake for 25-35 minutes or until done in the center. Use a toothpick to ensure it is not wet in the center.

Let cool in the pan for 30 minutes to 1 hour before cutting into squares. Store any leftovers covered at room temperature.