1. Line a 9 x 13-inch baking pan with foil; coat with cooking spray.To make skulls:
2. In a large glass bowl, combine marshmallows and butter. Microwave 2 minutes, stir, and microwave 30 seconds more, or until marshmallows are completely melted. Stir in rice cereal. Firmly press mixture into prepared pan. Let cool 30 minutes. Remove from pan, and cut skulls using a 3-inch cookie cutter. Press scraps together and cut more skulls. Insert a lollipop stick into the bottom of each skull. Press chocolate chips into skulls for eyes and nose, and use mini chips for teeth. Microwave vanilla frosting for 12 seconds. Stir and spoon onto top of each skull for a cap. Let dry completely.