Place cereal in lg bowl. Spray a 9" x 13" pan w/ cooking spray. In lg microwave-safe bowl, microwave 5 cups marshmellows, chocolate chips, corn syrup and butter for 2-3 mins, stirring 1/2 way through. It should be smooth, if not, microwave and stir until it is.
Pour mix over cereal. Toss w/ a lg spoon until completely coated. Stir in remaining marshmellows.
Press mix w/ back of spoon evenly into pan. Let stand 1 hr.