PREHEAT oven to 250ºF. Place peanut butter and butter in large microwaveable bowl. Microwave on MEDIUM-HIGH 1 min.; stir. Add marshmallows; microwave 1 min. or until marshmallows are puffed; stir until well blended. Add cereal, popcorn, and peanuts; mix well.

SPREAD evenly into two greased 15x10x3/4-inch baking pans.

BAKE 10 min. Cool completely. Break into small pieces.