Into large bowl, measure cereal. Butter 13x9-inch pan. In large microwavable bowl, microwave 5 cups marshmallows, the chocolate chips, corn syrup and butter uncovered on High 2 minutes to 3 minutes 30 seconds, 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 buttered back of spoon. Let stand uncovered at least 1 hour, or refrigerate if you prefer a firmer bar. For bars, cut into 6 rows by 4 rows. Store loosely covered at room temperature up to 2 days.