Microwave butter and peanut butter in large microwaveable bowl on MEDIUM-HIGH (70%) 1-1/2 min. or until butter is melted; stir until well blended. Add marshmallows; toss to coat. Microwave an additional 1-1/2 min. or until marshmallows are completely melted and mixture is well blended, stirring after 45 sec. Add remaining ingredients; mix well.
Press cereal mixture firmly into lightly greased 15x10x1-inch pan.
Refrigerate 1 hour or until firm. Cut into 32 bars to serve.