Break the bread into small crumbs, removing crusts. Heat together the milk, sugar and chocolate, and pour over the crumbs. Let stand for a few minutes, then add the beaten egg yolks and vanilla, and beat until the mixture is smooth.
Whip the whites stiff and fold into the first mixture, then pour all into a buttered mold, or the buttered upper part of a double boiler, and steam over boiling water for 1 hour. Turn out on a warm plate and serve while hot with a hard sauce, made by creaming 1/3 cup of butter with 1 cupful of powdered sugar, and 1 teaspoon each of vanilla and cream.