1Spray bottom and sides of 13x9-inch pan with cooking spray. In large microwavable bowl, microwave butter uncovered on High about 45 seconds or until melted.
2Add marshmallows; toss until coated. Microwave uncovered on High about 1 minute 30 seconds, stirring after 45 seconds, until mixture can be stirred smooth.
3Immediately add cereals; stir until evenly coated. Firmly press into pan. Cool about 1 hour or until firm.
4To make tiered cakes, using 3-, 2- and 1 1/4-inch round cutters, cut 8 rounds of each size; starting with 3-inch rounds. Stack rounds to form 8 (3-layer) bars. Decorate as desired, using decorating icing. Store loosely covered.