MICROWAVE butter in large microwaveable bowl on HIGH 30 to 45 sec. or until butter is melted. Add marshmallows; toss to coat. Microwave 1-1/2 min. or until marshmallows are completely melted and mixture is well blended, stirring after 45 sec.
ADD cereal; mix well. Press onto bottom of 13x9-inch pan sprayed with cooking spray. Cool completely. Cut into 54 bars. Meanwhile, combine MALLOW Bits and nuts.
MICROWAVE white chocolate in separate microwaveable bowl 1-1/2 min. or until chocolate is almost melted, stirring after 1 min. Stir until chocolate is completely melted. Add peanut butter; stir until melted.
PARTIALLY dip bars in chocolate mixture; gently shake off excess chocolate. Dip in MALLOW Bits mixture; place on parchment- or waxed paper-covered baking sheet.
REFRIGERATE 2 hours or until chocolate is firm.