Stove top: Melt butter in saucepan over low heat. Add marshmallows and stir until completely melted. Remove from heat and add cereal.
Microwave: Microwave butter until melted. Add marshmallows and microwave on high 3 minutes, stirring every minute.
Spread in buttered 9 x 13. Slightly dampen hands to press the mix and not have it stick.