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Boiled Frosting
2 cups sugar 3/4 cup water Stiffly beaten whites of 2 eggs Lemon or vanilla Boil sugar and water until it threads without stirring. Then gradually pour it into beaten eggs, beating mixture rapidly all the time. When all in, and mixture has thickened and is cool, it is ready for use in icing cakes. 3/4 cup sugar 2 squares chocolate
This is an Italian meringue under an older name, and the printed ingredient list has clearly been mangled in typesetting: the water is never quantified and "2 squares chocolate" belongs to a chocolate variation rather than to the plain lemon or vanilla frosting described. Work from the method, which is sound.
- The hinge: syrup temperature"Boils until it threads" is the thread or soft-ball boundary, roughly 230-238F on a thermometer. Below that the frosting stays soupy and will slide off the cake; much above it and the meringue sets grainy and stiff before it is spread.
- Missing waterNo quantity is given. For 3/4 cup sugar, enough water to dissolve it is about 3 to 4 tablespoons; the exact amount does not matter much because it boils off, which is why the thermometer governs rather than the clock.
- Sugar-to-white ratioThree-quarters of a cup of sugar for two whites is on the lean side by modern standards, which usually run closer to a cup. Expect a soft, marshmallowy frosting rather than one that holds sharp peaks.
- The stray chocolateTwo squares would generally have meant about two ounces of unsweetened baking chocolate, squares of that period commonly being an ounce apiece. Melted and beaten in at the end it makes a chocolate boiled frosting, but it is not part of the lemon or vanilla version above.
- Pouring the syrupAdd it in a thin steady stream down the side of the bowl, not onto the beaters, or it will spin into hard threads against the whisk. A stand mixer makes this a two-minute job that is awkward by hand.
- FlavouringThe recipe lists "Lemon or vanilla" without saying when to add it; best added after the meringue has cooled, since volatile flavour beaten into hot syrup largely dissipates. A teaspoon of vanilla or two of lemon juice is enough for this quantity.
- Egg whitesThe hot syrup heats but does not reliably pasteurise the whites. If serving to anyone frail, elderly, pregnant or very young, use pasteurised whites or powdered egg white.
- Working time and yieldEnough to frost the top and sides of one 8-inch layer, or the top of a sheet cake. Spread it as soon as it is cool, since it crusts over within the hour and cannot be smoothed once it does.
- WeatherBoiled frostings weep and refuse to set on a humid day. Cook the syrup a couple of degrees higher in damp weather, or choose a buttercream instead.
Boiled frosting, also sold in period cookbooks as boiled icing or white mountain frosting, is often said to have been the usual American cake covering from the mid-nineteenth century until buttercream displaced it, and the explanation traditionally offered for it is that it solved a real problem: uncooked confectioners' icing needed powdered sugar, which was reputedly expensive and inconsistent, while any household with granulated sugar and eggs could boil a syrup. The technique is the same one confectioners used for meringue work, and it depended entirely on a cook's ability to judge the thread, the soft ball and the crack by eye and finger before kitchen thermometers were common. Collections like this one printed the method in the barest possible shorthand because it was assumed knowledge; a chocolate variation's ingredients have ended up stranded at the foot of the plain recipe.