Place 75 g (2½ oz) butter in a pan, add 150 g (5 oz) of finely cut Mirepoix and cook to a light brown colour. Add the skin and chopped carcasses of the game under preparation; moisten with 3 dl (½ pt or 1¼ U.S. cups) white wine and reduce by two-thirds. Add 8 dl (1⅖ pt or 3½ U.S. cups) Sauce Demi-glace and allow to simmer gently for 45 minutes then pass firmly through a sieve so as to extract the maximum of flavour from the flavourings and carcasses.
Dilute the cullis obtained with 4 dl (14 fl oz or 1¾ U.S. cups) of stock in keeping with the game being prepared; simmer gently and skim carefully for 45 minutes so as to reduce the sauce by a good third.
Bring back the sauce to its correct consistency with the addition of mushroom cooking liquor and truffle essence. Pass through a fine strainer and finish with a little butter.