Microwave butter in 4-quart microwavable bowl on HIGH 45 seconds or until melted. Add marshmallows; mix to coat. Microwave 1-1/2 minutes or until marshmallows are melted and smooth, stirring after 45 seconds. Add cereal; mix to coat well.
Press firmly into greased foil-lined 13x9-inch pan. Cool; cut into squares.