Melt butter over medium heat in a medium skillet.

Add marshmallows and stir continuously until melted and smooth. Turn off heat.

Place cereal in a large bowl, pour marshmallow mixture over cereal and stir to combine.

Spray with cooking spray a 9x9 baking pan or dish. Pour in mixture and smooth out.

Let sit on the counter or refrigerate to set up. Cut into squares. Enjoy.