MICROWAVE butter in 4-qt. 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 and mixture is well blended, stirring after 45 sec.
ADD remaining ingredients; mix well. Press onto bottom of foil-lined 13x9-inch pan sprayed with cooking spray. Cool.