PLACE butter and peanut butter in large microwavable bowl. Microwave on MEDIUM (50%) 1 min.; stir.
ADD marshmallows; toss to coat. Microwave an additional 1-1/2 min. or until marshmallows are puffed; stir until well blended. Add cereal; mix well.
PRESS cereal mixture firmly into 9-in. square pan. Let stand 1 hour or until firm. Cut into 20 bars to serve.