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Measure cereal into large bowl. Butter rectangular pan, 13x9x2 inches. Microwave 5 cups marshmallows, the chocolate chips, corn syrup and margarine uncovered in large microwavable bowl 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 in bowl; quickly toss until completely coated. Stir in 1 cup marshmallows, saving 1 cup for the top. Press mixture evenly in pan, using buttered back of spoon. Let stand uncovered at least 1 hour, or refrigerate for a firmer bar. Cut into 2-inch squares. Store loosely covered at room temperature up to 2 days.