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Royal Frosting
1 cup sugar 1/4 teaspoon cream tartar 1 teaspoon lemon juice 1/3 cup cold water Boil the sugar, cream of tartar and cold water without stirring until it threads from spoon. Add the lemon juice. Beat until thick, and spread quickly.
A boiled sugar frosting with no egg white — essentially a small batch of fondant-style icing that sets hard and glossy. Everything depends on the syrup stage and on not stirring.
- "Threads from spoon"The thread stage, usually given as somewhere around 223-235F on a modern thermometer, though the book relies on the spoon test rather than a number. The syrup should fall from a lifted spoon in a thin thread rather than in drops; undercooked it will not set properly, and overcooked it tends toward gritty candy.
- Do not stirThe instruction is the hinge of the recipe. Any agitation while the syrup boils, or a single sugar crystal knocked down from the pan wall, seeds crystallization and the frosting goes sandy. Wash the sides down once with a wet brush and then leave it alone.
- Cream of tartarNot a leavener here. The acid inverts part of the sucrose into glucose and fructose during boiling, which interferes with crystal formation and keeps the finished frosting smooth. The lemon juice goes in off the heat, where it does little inverting and mainly supplies flavour and acidity.
- BeatingThe recipe says only to add the lemon juice and beat. Fondant practice is to let the syrup cool somewhat (often to around 110F) before beating, since beating while very hot tends to give coarser crystals. Beat until it turns opaque, thickens and loses its shine; a stand mixer takes a few minutes, a spoon considerably longer.
- Spread quicklyOnce it begins to set it sets fast and cannot be smoothed again. Have the cake ready and level before you start beating. If it stiffens in the bowl, a teaspoon of hot water beaten in will loosen it briefly.
- YieldRoughly three-quarters to one cup of frosting, enough for the top of one 8-inch layer or a thin coat over the tops of two. Double it for a filled two-layer cake.
- WeatherBoiled sugar frostings absorb moisture from the air. On a humid day they stay tacky and may weep; cook the syrup a couple of degrees higher than usual to compensate.
Boiled icings of this kind are often described as filling the gap between the older egg-white "royal icing" beaten raw with sugar and the butter-based frostings that took over American baking in the twentieth century, though that sequence is a tidy story rather than an established chronology. The name here is borrowed loosely: true royal icing is uncooked egg white and confectioner's sugar, while this is a cooked syrup beaten to a fondant. Vegetarian and reform cookbooks of the late nineteenth and early twentieth centuries circulated a great many such workarounds for recipes that avoid eggs, and cream of tartar was cheap, shelf-stable and already in every pantry for baking powder and boiled candy.