PREHEAT oven to 250°F. Grease 2 (13x9-inch) baking pans. Toss together cereal, popcorn and peanuts; set aside.

MICROWAVE butter and peanut butter in large microwaveable bowl on HIGH 1 min. or until butter is melted; stir until well blended. Add marshmallows; stir until evenly coated. Microwave an additional min. or until marshmallows puff. Stir in cereal mixture until well blended. Press half of the cereal mixture firmly into each of the prepared pans.

BAKE 10 min. Cool completely. Break into small pieces. Store in tightly covered container at room temperature.