Combine the chocolate and water in the top part of a double boiler. Cook over hot water until the chocolate melts, then slowly stir in the sugar and flour. Blend in the butter and vanilla.
Remove the mixture from the heat and add the well-beaten egg yolks. Beat the egg whites until stiff and fold them into the pudding.
Scrape the mixture into a well-buttered one-quart soufflé dish. Bake for one hour. Let the pudding cool in the mold, then chill if desired. The dessert will puff in the oven, then fall as it chills. Serve with mocha cream sauce.