Add ½ cup (125 milliliters) of white veal stock and ½ cup (125 milliliters) of mushroom cooking liquid to 3 cups (750 milliliters) of sauce velouté. Reduce the sauce to 3 cups (750 milliliters) and strain it through a fine chinois. Add 1 cup (250 milliliters) of Sauce Soubise I, and finish the sauce with 4 egg yolks and 4 ounces (125 grams) of butter. The sauce may need to be thinned with stock or heavy cream.