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 and mixture is well blended, stirring after 45 sec.
ADD cereal and nuts; mix well.
PRESS onto bottom of 13x9-inch pan sprayed with cooking spray. Cool completely before cutting into squares.