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Boiled Orange Frosting
Put 1 cup sugar and 1/4 cup water in a small saucepan. Stir until sugar is dissolved and boiling point is reached. Do not stir after it boils. Wash down sides of saucepan with pastry brush dipped in cold water to prevent formation of crystals. Cook until syrup spins a 4-inch thread when dropped from spoon held at least 8 inches above pan. Pour slowly onto 2 egg yolks beaten until thick and lemon colored, beating constantly with egg beater until mixture will hold its shape, then add Few gratings orange rind and 1/2 tablespoon orange juice and spread on cake 2 egg whites may be used instead of egg yolks if preferred.
A sugar-syrup frosting of the type that filled American cookbooks before buttercream took over. Everything depends on the syrup: cook it correctly and the rest is a two-minute job.
- The thread test in degreesA syrup that spins a thread from a spoon held high is somewhere in the thread-to-soft-ball range, roughly 230-240F on a candy thermometer, and a thread as long as 4 inches sits at the upper end of that. Use the thermometer and treat the thread as confirmation rather than the other way round.
- Why you must not stirOnce the syrup boils, a single sugar crystal dragged in from the pan wall can seed the whole batch and turn the frosting grainy. The wet pastry brush around the sides does the same work more safely; a lid clapped on for the first minute or two also washes the walls with steam.
- Pour slowly, and away from the beatersAim the syrup down the side of the bowl in a thin stream. Poured onto spinning beaters it flings against the cold bowl and sets into sugar shards that never dissolve.
- Egg temperatureTwo yolks are a small mass against a cup of boiling syrup, so the mixture generally comes up well past 160F, but it is not guaranteed and the finished frosting is never cooked again. If it is going to anyone very young, elderly, pregnant or immune-compromised, use pasteurized eggs.
- Yolks versus whitesThe yolk version is richer, deeper yellow, and sets softer. Two whites beaten to soft peaks give the familiar glossy white boiled icing and noticeably more volume, which matters if you are covering a layer cake.
- Add the juice last, and sparinglyHalf a tablespoon sounds mean but it is deliberate. More liquid slackens the frosting past the point where it holds a swirl; if you want more orange, add rind rather than juice.
- Yield and working timeEnough to top an 8- or 9-inch single layer, or to fill and thinly cover two. Spread it while it is still slightly warm, as it stiffens as it cools and cracks if you rework it.
- Equipment"Egg beater" means a rotary hand beater; a stand mixer on medium-high is the modern equivalent and makes the continuous beating far easier. A small heavy saucepan gives a syrup depth deep enough to read a thermometer.
Boiled frostings dominated American home baking from the late nineteenth century into the middle of the twentieth, and the phrasing here — the pastry brush on the pan walls, the thread measured from a spoon held eight inches up — is characteristic of the style of manual associated with the Boston Cooking School and Fannie Farmer's widely circulated books. Cooks needed a frosting that set firm enough to slice and keep on a sideboard for hours, which is exactly what a hot sugar syrup beaten into egg does and what a butter icing, in an unrefrigerated house, does not. The candy thermometer was already available when such recipes were written, but the spun thread survived in print for decades because it needed no equipment beyond a spoon and a steady hand.