Melt the butter, add flour, and pour on gradually, while stirring constantly, milk; cook until boiling-point is reached. Melt chocolate in a small saucepan placed over hot water, add sugar and water, and stir until smooth. Combine mixtures, and add yolks of eggs well beaten; cool. Fold in whites of eggs beaten stiff, and add vanilla. Turn into a buttered baking-dish, and bake in a moderate oven twenty-five minutes. Serve with Cream Sauce I.