Microwave butter in 4-qt. microwavable bowl on HIGH 45 seconds or until melted. Add marshmallows; toss to coat. Microwave 1-1/2 minutes or until marshmallows are completely melted and mixture is well blended, stirring after 45 seconds.
Add cereal; mix well.
Press firmly into greased foil-lined 13×9-inch pan. Cool. Cut into 24 squares.