In a microwave-safe bowl, melt butter and marshmallows in the microwave on HIGH for 3 minutes, stirring after 2 minutes, and stirring again when finished cooking, until smooth. Tint the mixture, if desired, with food coloring. Add Rice Krispies. Stir until well-coated and allow to cool slightly.
With well-buttered hands, shape the cereal mixture in the shape of mugs, reserving small portions of mixture for the handles. Attach the handle to the mug with frosting. Decorate the front of the mug as desired. The mugs are best eaten the day they're made.