Pick meat from the tails of twelve crawfish and set aside. Dry the shells and pound them all together in a mortar, adding 2 tablespoons good butter. Place in a sauce pan on moderate fire, stirring until it clarifies (about five minutes). Strain through a napkin, letting drop into cold water. When it congeals, take it out, place in warm basin, stir until it assumes the desired color. Mix with the picked meat, season highly with salt and cayenne, and serve on toast.