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MICROWAVE butter in large microwaveable bowl on HIGH 45 sec. or until melted. Add marshmallows; toss to coat. Microwave 1-1/2 min. or until marshmallows are completely melted, stirring after 45 sec.
STIR in peanut butter until blended. Add cereal and nuts; mix well. Press onto bottom of greased 13x9-inch pan.
MICROWAVE remaining ingredients in small microwaveable bowl 1 to 1-1/2 min. or until completely melted and mixture is well blended, stirring every 30 sec. Pour over cereal mixture; spread to cover. Refrigerate 1 hour or until chocolate mixture is firm.