Line a 9 x13 pan with aluminum foil and spray well with non-stick cooking spray.
In a large pot, melt butter on medium-low heat. Add marshmallows stirring frequently, until almost all the way melted but not quite.
Take the mixture off heat and stir in cereal until coated.
Pour mixture into prepared pan and press down evenly. Press mixture down firm enough that the bars will hold together. Allow bars to cool completely before cutting into squares.