Start by melting the butter and marshmallows together in a microwave-safe bowl, it took me about a minute and a half to melt completely. Next, add the cereal until it's well combined and every piece of cereal has the marshmallow mixture on it. Spread the mixture into a greased 8x8 baking pan and let set up for an about an hour before eating (this step is optional and you can eat them right away if you enjoy gooey bars).