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Boiled Frosting
1 cup sugar, 1/2 cup water, Unbeaten white 1 egg, 1/2 teaspoonful vanilla. Put sugar and water in saucepan, bring to the boiling point, and let boil vigorously, without stirring, until syrup will spin a thread when dropped from tip of spoon. Pour syrup gradually, while beating constantly (using an egg beater), on white of egg. Remove egg beater and continue the beating (using a spoon) until of the right consistency to spread; then add vanilla.
This is an Italian meringue in miniature: hot sugar syrup whipped into egg white. The whole recipe turns on the syrup, so read that part first.
- "Spin a thread"Roughly the thread stage, in the region of 230-235F on a candy thermometer; cooked a little further, towards soft ball, the frosting sets firmer. Undercooked, the icing never sets; much above it turns grainy and hard as it cools.
- Do not stirStirring after the sugar dissolves invites crystallization. If crystals form on the pan sides, wash them down with a wet pastry brush rather than touching the syrup.
- Unbeaten whiteThe original pours syrup onto a completely unbeaten white, which works but takes far longer to build volume. Beating the white to soft peaks first, then adding the syrup in a thin stream, is easier and gives the same result.
- EquipmentAn "egg beater" here is a hand-cranked rotary beater. A stand mixer on medium-high does the job with no need for the spoon; the original's switch to a spoon is unexplained, though the likeliest reason is that the crank grew too heavy as the frosting stiffened.
- Pour positionAdd the syrup down the side of the bowl, not onto the beaters, or it will fling hardened threads of sugar around the bowl instead of dissolving into the white.
- Raw eggThe syrup cooks the white only partially. If the frosting is for anyone who should avoid undercooked egg, use pasteurized whites, though they may take longer to whip up.
- HumidityBoiled frosting is unforgiving on a damp day; it stays tacky and weeps. Make it in dry conditions and spread it as soon as it holds a shape, since it firms quickly in the bowl.
- YieldRoughly enough to cover the top and sides of a single 8- or 9-inch layer, or the tops of two; double it for a filled two-layer cake.
The phrasing and the method match what students were taught in Boston kitchens around 1900. Boiled frosting was widely used as a cake covering in that period, valued because it set glossy and held its shape without the quantity of butter a modern buttercream demands. The instruction to boil "until syrup will spin a thread" reflects a period when candy thermometers were far from universal in home kitchens and cooks judged sugar by eye, from thread through soft ball to crack. Directions of this kind appear across nineteenth-century American cookbooks, and the vocabulary long outlived the need for it.