In a large saucepan, melt the butter over low heat. Add the marshmallows and stir until melted. Remove from the heat, then add the rice cereal and stir until well coated.
Take small amounts of the mixture and fill both sides of a plastic egg, slightly over-filling one side. If the mixture is too sticky, you can spray your hands with a light cooking spray. Press a chocolate egg in the center on one side of the egg, then close the plastic egg to shape it. (It should be full enough to meet with a little resistance as you close it.) Gently release the rice cereal egg from the mold, decorate with your choice of sprinkles and set aside in an egg crate until set. Repeat with the remaining rice cereal mixture and chocolate eggs.