1. Into a large bowl, measure cereal. Butter a 13 x 9-inch pan. In a large microwaveable bowl, microwave the 5 cups of marshmallows, the chocolate chips, corn syrup and butter uncovered on High 2 minutes to 3 minures and 30 seconds, stirring after every minute, until melted and smooth when stirred. Stir in vanilla.
2. Pour over cereal; quickly toss until completely coated. Stir in 1 cup marshmallows.
3. 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 squares, cut into 6 rows by 4 rows. Store loosely covered at room temperature up to 2 days.
Not a NESTLÉ®-tested recipe.