Beat the butter and sugar together to a cream, beating until light and very white; add the whites of the eggs, beaten to a stiff froth, putting in one at a time; whip these well together, and add the boiling water and the Brandy. Set over the fire in a bain-marie (boiling water) and stir till creamy and no longer. Serve either hot or cold. Do not let it boil. This sauce is excellent with Blackberry Roll, Bread Puddings, etc.