Spray the bottom and sides of a 13-by-9-inch pan with nonstick cooking spray.
Place the butter and marshmallows in a large microwave-safe bowl. Microwave uncovered on high for about 2 minutes, stirring after each minute, until smooth.
Immediately stir in the cereal until evenly coated. Transfer the mixture to the pan, press it into an even layer with the buttered back of a spoon, and cool for at least 10 minutes.
Cut the cereal mixture into 6 rows by 4 rows to make 24 bars, then store them in a loosely covered container.
Notes
Use pot holders when removing the hot bowl from the microwave.