Microwave butter and peanut butter in large microwaveable bowl on MEDIUM-HIGH (70%) 1 ½ minutes 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.