Spray 4-qt. microwaveable bowl and two 8- or 9-inch round cake pans with cooking spray; set aside. Microwave butter in prepared bowl on HIGH 45 sec. or until melted. Add marshmallows; toss to coat. Microwave 1-1/2 min. or until marshmallows are completely melted, stirring after 45 sec.
Add cereal; mix well. Press half of the cereal mixture firmly and evenly into each of the prepared cake pans. Cool completely. Remove from pans. Cut ears from one of the rounds; use section remaining after cutting ears for bow tie. Leave second cereal round whole. For ease in cutting, first measure and mark distances with toothpicks, then cut between toothpicks with a sharp knife. Arrange cereal treat pieces on a large (at least 21×12-inch) tray, cutting board, baking sheet or foil-covered cardboard.