MICROWAVE butter in large microwaveable bowl on HIGH 45 seconds or until melted. Add marshmallows; toss to coat. Microwave 1-1/2 minutes or until marshmallows are completely melted, stirring after 45 seconds.
STIR in peanut butter until blended. Add cereal and peanuts; mix well. Press mixture firmly into greased 13x9-inch pan.
MICROWAVE chocolate chunks and butterscotch chips in small microwaveable bowl 1 to 1-1/2 minutes or until chocolate is completely melted and mixture is well blended, stirring every 30 seconds. Pour over cereal mixture; spread to cover. Refrigerate 1 hour or until chocolate mixture is set. Cut into 36 squares.