Mince chicken finely, gradually add butter and egg yolk, season well, sieve if desired. Stir in half-whipped cream, fold in stiffly-beaten egg whites. Place in a well-buttered soufflé mould or straight-sided tin; cover with greased paper and steam gently and evenly 50–60 min. (Individual moulds 25 min.)
Turn out on to a hot dish, coat quickly with sauce. Garnish with mushrooms and serve at once.