Line a 9x9-inch baking pan with foil and spray with cooking spray. Set aside.
Melt the butter and the marshmallows in a medium sized saucepan over medium-low heat. As the marshmallows begin to melt, slowly add in the brownie mix until it is incorporated with the marshmallows and butter. Quickly stir in the cereal.
Pour the marshmallow-cereal mixture into the prepared baking dish. Press to flatten. (I like to spray my hands with cooking spray and use them to press the treats into the pan – just be careful not to burn yourself.) Cool completely before cutting into squares.