Boiled Icing

This is Italian meringue under an older name: hot sugar syrup beaten into egg whites until the mass turns glossy and holds a peak. The whole recipe hinges on the syrup, and a thermometer settles in seconds what the fork test leaves to judgment.

• What "ropes" meansThe syrup should spin a thread from the fork rather than drip. That is usually put somewhere around 230-240F on a candy thermometer, spanning thread to soft-ball stage. Much cooler and the icing tends to stay soupy; much above 245F it can set hard and grainy before it can be spread.
• Pour thin and steadyAdd the syrup down the side of the bowl in a slow stream with the beater running. Dumping it into the centre of the whites throws hot sugar onto the whisk, where it flings out and hardens into threads.
• Whites first, then syrupHave the whites already at stiff peaks before the syrup reaches temperature. Whites that sit waiting for a slow syrup will weep, but a syrup that waits for the whites overshoots quickly.
• A pinch of cream of tartarAn eighth of a teaspoon beaten into the whites at the froth stage gives a noticeably more stable foam. A copper bowl has a similar stabilising effect.
• Beat until coolKeep beating after the last of the syrup goes in, three to five minutes with a stand mixer, until the outside of the bowl is no more than warm. Icing spread while hot slides off the cake.
• Use at onceThe instruction is meant literally. Boiled icing crusts quickly and does not come back once it has; have the cake cooled and on its stand before starting the syrup.
• HumiditySugar meringue draws water from the air. On a damp day the icing stays tacky and may bead; cook the syrup two or three degrees higher to compensate.
• YieldRoughly two to three cups of icing, enough to cover the top and sides of a single 8-inch cake or a batch of cupcakes.
• FlavoringA teaspoon of vanilla, or half that of almond extract, is a usual proportion for this quantity. Add it at the end, as the recipe directs, once the beating is nearly done.