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Boiled Frosting
1 cup sugar
1 egg-white
½ cup water
½ 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° 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.
This is what is now usually called boiled icing or seven-minute frosting's stovetop cousin — an Italian meringue made with a single egg white. The hinge is the syrup temperature; everything else follows from it.
- 238°F is the number that mattersIt sits inside the soft-ball range (usually given as about 235-240°F), and since the recipe supplies the figure, a candy or instant-read thermometer is easier to judge than the spoon-thread test. Cooked much cooler, the frosting tends to stay slack; cooked much hotter, it can turn grainy and stiffen before you can spread it.
- Ignore "cool while beating the egg-white"Modern practice is to have the white already beaten to soft peaks before the syrup is ready, then pour the syrup in while it is still hot — around 235-240°F. Letting the syrup cool first, as written, gives a weaker meringue and does far less to cook the white.
- Do not stir after the sugar dissolvesOnce the syrup is clear, leave it alone. Stirring drags sugar crystals back into the solution and seeds graininess. Brush any crystals off the sides of the pan with a wet pastry brush, or cover the pan for the first minute so condensing steam washes them down.
- Egg white sizeEgg sizes were not standardised in this period, so the original white need not have matched a modern one. One large white (about 1 fl oz / 30 g) suits a cup of sugar; if the frosting seems slack, suspect the syrup temperature before the egg.
- Pour in a thin stream, down the sideAim between the beater and the bowl wall so the syrup does not hit the whisk and set into hard threads. A stand mixer on medium-high makes this a two-minute job; by hand, expect five to eight minutes of steady beating until the bowl is barely warm and the mixture holds a peak.
- WeatherBoiled frosting is unreliable on a humid day — it absorbs moisture from the air and weeps. Cook the syrup two degrees higher in damp weather, and frost the cake the day it is served.
- Yield and handlingA modest batch, better suited to topping a single layer or sheet than to filling and covering a two-layer cake. Spread it as soon as it is thick enough; it crusts as it stands and cracks if worked after that.
- Raw-egg noteHot syrup raises the white's temperature substantially but not always to a reliable 160°F, especially in a single-white batch. If you are serving anyone who should avoid undercooked egg, use pasteurized whites.
Boiled frosting belongs to the era when American cake decorating ran on sugar syrup and a strong arm rather than butter. Before creamed buttercream became the default — and before household refrigeration made a butter-heavy icing practical to keep — the finishes often used for a layer cake were a raw "water icing" of confectioners' sugar, or this: a meringue set by boiled syrup, glossy, snow-white and quick to crust. The Butterick book gives both the older sensory cue (the syrup "will form a thread") and a precise figure of 238°F beside it. The same technique, scaled up and enriched with butter, is the Italian meringue buttercream of modern pastry work; the near relation cooked over hot water instead of on the stovetop became known as seven-minute frosting.