Beat the butter to a cream, and gradually add the sugar, beating constantly. When very light, add the yolks of the eggs, and beat till these are very light. Then add the milk and the Chocolate, which you will have dissolved in four tablespoonfuls of boiling water. Mix thoroughly, and add by degrees the flour, beating all the while. Give a very vigorous beating, and add the whites of the eggs, beaten to a stiff froth. Stir them very carefully into the batter, and add the Vanilla. Mix all very quickly and lightly, and set to bake in buttered pan in a moderate oven for three-quarters of an hour.