In a large sauce pan, or a double boiler, melt the butter over low heat. When the butter's completely melted, add in the marshmallows and stir continuously, until they're completely melted and the mixture is smooth. completely melted; Remove the pan from heat. Spray a spatula, or a large spoon, with non-stick cooking spray to prevent sticking. Add the cereal and melted marshmallows to a large bowl, and using the coated spatula, stir until evenly combined and the cereal's well coated. Transfer the coated cereal to a 9x13 inch baking dish, lightly sprayed with cooking spray. Use the prepared spatula to gently press the mixture evenly into the baking dish. When the cereal has completely cooled, spread the frosting out in an even layer over top. Sprinkle sprinkles, and cut into squares to serve.