1Spray bottom and sides of 9-inch square 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 and peanuts; stir until evenly coated. Press firmly into pan. Cool about 1 hour or until firm. Cut into 9 blocks, 3 rows by 3 rows. Decorate as desired, using decorating icing. Store loosely covered.