Spray 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. With pot holders, remove bowl from microwave.
Immediately stir in cereal until evenly coated. Press mixture evenly into pan using buttered back of spoon. Cool at least 10 minutes.
With table knife, cut into 6 rows by 4 rows. Store in a loosely covered container.