PLACE 2 Tbsp. butter and 1/2 cup Kraft Peanut Butter in large microwavable bowl. Microwave on MEDIUM 1 minute; stir.

ADD 4 cups Jet-Puffed Mini Marshmallows; toss to coat. Microwave an additional 1-1/2 min. or until marshmallows are puffed; stir until well blended. Add 6 cups banana nut whole grain cereal flakes; mix well.

PRESS cereal mixture firmly into 9-inch square pan. Let stand 1 hour or until set before cutting into 20 bars to serve.