1In large microwavable bowl, mix cereals.
2In 2-cup microwavable measuring cup, microwave butter uncovered on High about 30 seconds or until melted. Stir in brown sugar, vanilla and nutmeg. Microwave uncovered on High about 30 seconds or until hot; stir. Pour over cereal mixture, stirring until evenly coated. Microwave uncovered on High 2 to 3 minutes, stirring every minute until mixture just begins to brown. Spread on waxed paper or foil.
3In small microwavable bowl, heat frosting uncovered on High about 15 to 20 seconds, just until melted. Drizzle frosting over cereal mixture. Refrigerate 15 to 20 minutes or until set.