Prepare the vol-au-vent as directed. Put one quart of oysters on to boil in their own liquor. As soon as a scum, rises, skim it off, and drain the oysters. Return half a pint of the oyster liquor to the sauce-pan. Mix two heaping tablespoonfuls of butter with a scant one of flour, and when light and creamy, gradually turn on it the boiling oyster liquor. Season well with salt, pepper and, if you like, a little nutmeg or mace, (it must be only a "shadow"). Boil up once, and add three tablespoonfuls of cream and the oysters. Stir over the fire for half a minute. Fill the case, cover, and serve immediately.