Melt butter in a small saucepan. Saute mushrooms for 2 to 3 minutes. Sprinkle
flour over mushrooms and butter and stir one minute. Add the light cream and
cook over medium heat, stirring constantly until sauce boils. Stir one
tablespoon sauce into beaten egg yolk. Add egg yolk mixture to sauce and
continue cooking, stirring, until sauce is desired thickness. DO NOT BOIL.
Season with sherry, lemon juice, salt, pepper, and paprika to taste. Stir in
minced truffle if desired and remove from heat.