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Boiled Icing
Boil one cup of granulated sugar with one-fourth cup of water, until the syrup hairs when dropped from a spoon. Have ready the beaten white of one egg. Pour the syrup slowly upon the egg, stirring constantly. Flavor the same as the cake and spread on the cold cake, when the icing is stiff enough not to run. Cut in squares or slices.
A one-egg boiled icing, essentially a small Italian meringue. The whole thing turns on the syrup temperature, which the original describes by eye rather than by number.
- "Hairs when dropped from a spoon"The thread stage, when a drip from the spoon trails a fine filament rather than falling in drops. On a candy thermometer that is about 230-235F; many later boiled-icing recipes push to 238-240F (soft ball) for a firmer set. At the lower end the icing stays glossier and softer, at the higher end it holds peaks and dries to a thin crust.
- ScaleOne cup sugar, a quarter cup water and one white makes a modest amount: roughly enough for the top and sides of a single small layer, or the top of a small sheet cake. Double it for a two-layer cake.
- Beat the white first'Have ready the beaten white' indicates the white should be beaten before the syrup is done. Soft peaks rather than a white beaten dry will take up the stream of syrup more readily.
- Pour thin and off the beaterA slow, steady thread down the side of the bowl. Poured onto the whisk itself the syrup flings into threads on the cold bowl wall and is lost. Keep beating until the mixing bowl is no more than warm to the hand, which takes several minutes with a mixer and longer by hand.
- Flavoring'The same as the cake' means whatever flavored the cake, typically vanilla, lemon, almond or rose water. Adding it at the end, once the icing has cooled, keeps the volatile flavor from being driven off by the hot syrup.
- Cold cake, and work fastBoiled icing sets as it cools and becomes unspreadable within minutes of reaching the right consistency. Have the cake trimmed and on its plate before the syrup goes on the boil. If it stiffens too far, a teaspoon of hot water beaten in will loosen it.
- HumiditySugar meringues are hygroscopic. On a damp day the icing will weep and stay tacky no matter how well it was made; cook the syrup a couple of degrees higher to compensate.
- Egg safetyThe hot syrup brings the white well up in temperature but does not reliably pasteurize it, and the icing is not baked afterward. Anyone cooking for young children, pregnant women or the immunocompromised should use pasteurized whites.
Boiled icing, sometimes called white mountain icing, was a common American cake covering in the nineteenth and early twentieth centuries. The technique is the same one that produces Italian meringue, and cooks judged the syrup by hand long before thermometers were common kitchen equipment "hairs," "threads," "soft ball" and "crack" were a shared vocabulary for stages of sugar. The closing instruction to cut in squares or slices suggests this was written with a plain sheet or loaf cake in mind rather than a decorated layer cake.