Measure the cereal into a large bowl. Grease a 13x9-inch pan with non-stick cooking spray. In another large microwavable bowl, microwave 5 cups marshmallows, the chocolate chips, corn syrup and butter uncovered on High for 2-3 minutes, stirring after every minute, until melted and smooth when stirred. Stir in vanilla.
Pour over cereal; quickly toss until completely coated. Stir in 1 cup marshmallows.
Press mixture evenly in pan, using back of spoon (or your hands- sometimes it's just easier that way). Let stand uncovered at least 1 hour, or refrigerate if you prefer a firmer bar. Store loosely covered at room temperature up to 2 days.
Notes
You could definitely make the marshmallow and chocolate chip mixture over the stove top. I prefer to do it this way, because I find that it helps it cooks more evenly when I can stir it continually throughout cooking.