Foaming Sauce

A warm dessert sauce for steamed and boiled puddings — essentially hard sauce taken to the edge of melting rather than served cold. The whole trick is heat control.

• The hinge: gentle heat onlyThe water in the pan should be hot but never boiling. If the butter is allowed to melt outright, the mixture will separate into a greasy pool instead of holding as a foam, and there is no egg here to bind it back together.
• What a teacup meansIn American books of this period a teacup is usually reckoned at about three-quarters of a cup, though it is genuinely inconsistent from book to book. Half a teacup of butter would then be about 6 tablespoons; use equal volumes of butter and sugar and the proportion holds however you read it.
• Use fine sugarSugar of the 1870s varied a good deal in fineness, and the sauce benefits from superfine or sifted confectioners' sugar, which dissolves in the warm butter instead of staying gritty.
• "Beat to a froth" means cold butter work"Beat to a froth" means the beating comes first -- Cream the softened butter and sugar thoroughly before any heat touches it — a stand mixer for three or four minutes, until pale and visibly aerated. The heat only softens and expands what you have already beaten in.
• Stirring one wayA common direction in nineteenth-century cookery and a matter of habit rather than physics. What matters is steady, continuous stirring so no part of the mixture outruns the rest and breaks.
• The tablespoon of hot waterIt thins the sauce to a pourable foam and helps dissolve the last of the sugar. The recipe offers vanilla in its place; spirits such as sherry or brandy appear as flavourings in comparable period sauces.
• Yield and servingRoughly three-quarters of a cup, enough for four to six servings of pudding. Make it while the pudding is unmoulding and serve at once; it will not sit well, and reheating risks breaking it.