Cook the macaroni in boiling salted water until tender. Drain and rinse in hot water. Combine the milk, mustard, salt, pepper and 2 cups of the cheddar cheese in the top of a double boiler and cook until the cheddar is melted and sauce is smooth, stirring frequently. Add the macaroni and blend well. Split the frankfurters lengthwise, without separating them, and fill with the macaroni mixture. Sprinkle with the remaining cheddar and bake in a hot oven (400 degrees F.) for 15 minutes. Serve hot.