Grease or spray bottom and sides of 13x9-inch pan. In large bowl, measure cereals.
In large microwavable bowl, microwave butter uncovered on High 30 to 45 seconds or until melted. Stir in 7 1/2 cups of the marshmallows; microwave uncovered on High 45 seconds. Stir; microwave 30 to 45 seconds longer or until mixture can be stirred smooth.
Working quickly, pour marshmallow mixture over cereals in bowl, using spoon to fully coat. Add remaining 1 1/2 cups marshmallows, mixing to incorporate.
With greased hands, firmly press mixture in pan. Place melted chocolate in small resealable food-storage bag; cut off small corner of bag, and drizzle over bars.
Cool 1 hour. For 24 bars, cut into 6 rows by 4 rows. Store tightly covered up to 1 week.