Prepare one pound of puff paste and roll it out to about two and one-half inches in thickness. Cut a round out of the paste with a tin cutter, brush the round over with a paste brush dipped in beaten egg without touching the sides, and with a knife, the point being held inwards towards the center, make a circular incision pressing the inner paste to prevent its closing up again. Make a lid with some of the remaining paste to fit in the wall of the vol-au-vent, brush the top over with beaten egg, place the paste on a baking-sheet and bake for half an hour in a moderate oven. Put one ounce of butter and a tablespoonful of flour in a stewpan, stir it over the fire until well mixed, then pour in gradually one pint of stock and keep stirring over the fire until boiling. Put one wineglassful of sherry, two tablespoonfuls of mushroom ketchup and a little cayenne pepper in the sauce, move it to the side of the fire and let it simmer gently for fifteen minutes for the purpose of clarifying it; then skim it, put in a piece of glaze, put it on the fire and let it boil quickly for five minutes longer. When the glaze has dissolved, strain the sauce through a fine hair-sieve into another saucepan, put in it two partially-boiled calf's sweetbreads, three or four cockscombs, a few truffles and mushrooms, all cut up into fairly small pieces. Boil the sauce gently until the sweetbreads, etc., are cooked, then season to suit the taste with pepper and salt. When cooked take the vol-au-vent out of the oven, remove the center carefully without damaging the case, put the above mixture into it and then cover with the lid. Place the vol-au-vent on a folded napkin on a hot dish and serve while very hot. If the cases are made and cooked before the mixture with which to fill them is ready, they can always be placed in an oven and warmed again.