Preheat the oven to 350 degrees F. Spray a 9-inch square baking pan with nonstick cooking spray. Line the pan with parchment paper and spray again.
Prepare the brownie batter according to the package instructions. Gently fold in 3 cups of the snack mix. Pour the batter into the prepared pan and spread to the edges. Sprinkle the top of the batter with the remaining snack mix.
Bake until the brownies are set, the surface is crackly and the snack mix is golden brown, 35 to 40 minutes. Let the brownies cool completely before slicing.