In a large microwave safe bowl, heat the 3 tablespoons of butter and marshmallows on high for 2-3 minutes. Stir until completely melted.
Add in the rice krispie cereal, and mix until the cereal is well coated.
Lightly spray a 9×13 baking pan with cooking spray. Pour the rice krispie mixture into the baking pan, and press down. Refrigerate for at least 30 minutes.
Cut the rice kirspies into triangles.
Icing
Mix together the icing ingredients until smooth. Spoon into a piping bag. Pipe the icing onto the rice krispies and decorate with the round candies.