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Boiled Frosting
1 cup sugar 1/2 cup water 1 egg-white 1/2 teaspoon vanilla Cook the sugar and water together, stirring until the sugar has dissolved. Then, without stirring, cook until the sirup will form a thread when dropped from the tip of a spoon (or until it reaches 238 degrees F.). Remove from the fire and cool while beating the egg-white stiff, then pour the sirup in a thin stream over the stiff white, beating the mixture constantly until thick enough to spread.
A one-egg-white Italian meringue, and the whole thing rests on syrup temperature. A candy thermometer takes all the guesswork out of the "thread" test.
- 238F is the number that mattersThat sits within the soft-ball stage (roughly 235-240F). Much below it and the frosting tends to stay soupy and never set; much above it and it can turn stiff and grainy before you can spread it.
- The thread testThe old visual cue is a syrup that falls from the spoon in a fine unbroken thread rather than in drops. It is workable but imprecise; use the thermometer and treat the thread as confirmation.
- Do not stir after it boilsStirring past the dissolving stage drags sugar crystals down from the pan sides and seeds the whole batch, which is what makes boiled frosting turn sandy. Wash the sides down once with a wet pastry brush and then leave it alone.
- "Cool while beating the egg-white"This means only the minute or two it takes to whip the white, not a real cooling. The syrup should still be hot enough to pour freely, since it is the heat that cooks and stabilises the meringue.
- PouringAim the thin stream down the side of the bowl, not onto the beaters, or it will spin into hard sugar threads. A stand mixer on medium-high makes this a one-person job.
- How long to beatExpect five to eight minutes. The frosting is ready when the bowl is only just warm and the mixture holds a soft peak and its own shape on a spoon.
- Add vanilla at the endBeat it in once the mixture has cooled somewhat, so the flavour is not driven off by the hot syrup.
- Yield and timingOne egg white makes a modest amount: enough to top about a dozen cupcakes, or to cover a single layer. Use it soon after making, as it crusts quickly and cannot be rewarmed. It is slower to set on a very humid day.