Place a steamer rack in a large stock pot and add water to a depth of 1/2 inch below rack. Add vinegar and mustard; bring to a boil. Grasp each lobster just behind the eyes and place on rack. Cover with a tight fitting lid. Steam over high heat 12 minutes. (Do not remove lid during steaming procedure.) Lobster will be bright red when done. Remove lobsters from stock pot with kitchen tongs. Reserve stock for use in other recipes.
Serve whole lobsters immediately with clarified butter or remove meat for use in other recipes. (See how-to procedure for removing meat from shell).