Butter rectangular pan, 13x9x2 inches. Place marshmallows and butter in large microwavable bowl. Microwave uncovered on High about 1 minute or until mixture can be stirred smooth. Stir in cereal. Press in pan.
Let stand at room temperature about 1 hour or until firm. Cut into 6 rows by 4 rows. Store tightly covered at room temperature up to 2 days.