Grease a 9x13-inch pan with cooking spray. Set aside.

Melt the butter and marshmallows together in a large pan over medium-low heat, stirring constantly until just melted.

Pour the marshmallow mixture over the cereal.

Fold the cereal and marshmallow mixture together.

Empty the mixture into the greased pan and press evenly. (Be gentle. Don't pack the cereal in hard.)

Cool completely and cut into bars. Store any leftovers inside the pan, covered tightly with foil or plastic wrap, for up to 5-7 days.