Spray bottom and sides of 8- or 9-inch square 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.
Immediately stir in cereal until evenly coated. Pour cereal mixture into pan. With buttered back of spoon, press mixture in pan until even. Cool at least 10 minutes.
With table knife, cut cereal mixture into 4 rows by 4 rows to make 16 bars.