Measure cereal into large bowl. Butter 9×13-inch rectangular pan (please, would a 9×13-inch pan be square, uh, I don’t think so). Melt together the 5 cups marshmallows the corn syrup, margarine, and chocolate chips in 3-quart saucepan over low heat, stirring constantly; remove from heat. Stir in vanilla. Pour over cereal in large bowl. Mix quickly until completely coated with chocolate. Stir in 1 cup marshmallows.

Make squares by pressing mixture evenly in pan with butter back of spoon. Let stand at least one hour, or refrigerate if a firmer bar is desired. Cut into 2-inch squares.