Frosting (Pound brown sugar enough water)

A three-ingredient boiled brown sugar frosting, closer to a penuche than to a buttercream. The whole result depends on the sugar temperature and on beating it at the right moment.

• Boil to a threadThe thread stage is usually given as around 230-235F on a candy thermometer, the point where syrup lifted on a spoon falls in a fine thread, though older manuals place it variously. Frosting boiled only this far stays soft and glossy and will run on a warm cake; taken further, to the soft-ball stage (about 235-240F), it will set firm enough to slice.
• Butter the size of an eggThat is about 2 ounces, or 4 tablespoons. Stir it in off the heat, after the syrup has stopped boiling, so it emulsifies rather than separating into an oily film.
• Enough water to dissolveFor a pound of brown sugar, roughly 2 1/4 cups packed, about 1/2 cup water is enough. The exact amount does not matter much since it boils off; the thermometer, not the liquid, tells you when to stop.
• Brown sugarLight brown gives a caramel-toffee frosting, dark brown a pronounced molasses flavour that can turn slightly bitter. Nineteenth-century brown sugar was less uniform than today's; light brown is the closer match.
• Beat after cookedThis is the hinge. Let the syrup cool to about 110F, undisturbed, then beat until it loses its shine and thickens. Beating while it is still hot gives a thin glaze; beating too long past the change in gloss gives a grainy mass that seizes in the bowl.
• Add the vanilla lateStir the teaspoon of vanilla in with the butter or during beating, not into boiling syrup, where the alcohol carries most of the aroma off.
• Working timeOnce it thickens it sets quickly. Have the cake on its stand and an offset spatula ready, and spread in as few strokes as possible; if it stiffens, a teaspoon of hot water or cream beaten in will loosen it.
• YieldEnough to fill and cover a two-layer 8- or 9-inch cake, or to top a 9x13 sheet generously.