1Spray bottom and sides of 13x9-inch pan with cooking spray. In large microwavable bowl, microwave butter and marshmallows uncovered on High about 2 minutes, stirring after each minute, until mixture is smooth. Remove from microwave.
2Immediately stir in cereal until evenly coated. Pour cereal mixture into pan. With buttered back of a spoon, press mixture in pan until even. Cool at least 10 minutes.
3With table knife, cut cereal mixture into 6 rows by 4 rows to make 24 bars. Store in loosely covered container.