1Butter bottom and sides of 13x9-inch pan.
2In large microwavable bowl, microwave marshmallows and butter uncovered on High about 2 minutes, stirring after every minute, until smooth.
3Immediately stir in cereal until evenly coated. Press in pan. Cool about 30 minutes or until firm. For bars, cut into 6 rows by 4 rows. Store loosely covered.