Melt chocolate in top of double boiler. Beat egg yolks, add sugar gradually, beat. Add yolks and milk to melted chocolate and stir until smooth. Pour into bowl and cool. Beat egg whites until stiff and fold into chocolate mixture. Whip cream and fold into mixture with vanilla and cognac. Pour into 2 quart soufflé dish. Chill thoroughly. Sprinkle top with sifted cocoa. Serves 6 to 8.