Boiled Frosting
1 cup sugar 1/3 cup water 1/8 teaspoon cream tartar 1 egg white Boil the water, sugar and cream tartar till it forms a soft ball in cold water; pour in a fine stream on the egg white beaten very stiff, beat as you pour; continue beating until stiff and smooth. Spread on a cold cake. Dip knife in warm water.
Chocolate Filling, No. 1 1 cup sugar 2 squares chocolate, grated 2 tablespoons butter 3/4 cup milk 2 tablespoons corn starch in 1/4 cup milk 1 teaspoon vanilla Boil first four ingredients together for five minutes, add corn starch in milk and boil for three minutes more. Beat until cool and add vanilla. Chocolate Filling, No. 2 1 egg yolk 1/2 cup sweet milk 1/4 cake chocolate 1 cup sugar Flavor with vanilla and cook like custard.
Chocolate Filling, No. 3 Melt 1/4 pound Baker's chocolate over warm water and stir it into twice the rule for boiled icing. Spread while warm, as it stiffens quickly. If too firm, beat in slowly sweet cream until as thin as desired.
This is Italian meringue under an older name, followed by three fillings of increasing richness. The page assumes you know the cold-water test; a thermometer removes the guesswork.
- Soft ball, in numbersBoil the syrup to 238-240F. Below that the frosting stays soupy and weeps; above about 245F it sets hard and dull before you can spread it.
- Pour, do not dumpThe hinge of the whole thing: the syrup must go in a thin thread down the side of the bowl onto whipped whites while the beater runs. Poured fast, or onto the whisk, it either cooks the whites into threads or sinks to the bottom as a candy layer.
- Cream of tartarAn eighth teaspoon is there to invert some of the sugar and keep the syrup from crystallizing on the pan walls. Do not skip it, and do not stir the syrup once it boils.
- Egg white safetySyrup at 240F warms the meringue but does not reliably pasteurize it, and the frosting is never baked. Anyone cooking for the very young, the elderly, or pregnant or immunocompromised eaters should use pasteurized whites.
- "2 squares chocolate"Baker's unsweetened chocolate was commonly sold as a scored cake broken into one-ounce squares, so two squares is most likely 2 oz unsweetened, though cake sizes varied. "1/4 cake" in Filling No. 2 probably means about 2 oz as well, but the wording is loose enough that it is worth reading as an approximation.
- "Sweet milk"Plain fresh whole milk, named to distinguish it from sour milk and buttermilk. Ordinary whole milk is the equivalent.
- Cornstarch fillingSlake the starch in the cold quarter cup of milk before it goes in, whisk constantly, and hold the three minutes of boiling. Less than that leaves a raw starch taste; the mixture will look thick well before the starch has finished cooking.
- Filling No. 2"Cook like custard" means a double boiler or very low direct heat, stirred until it coats a spoon at about 175-180F. It will not set firm; it is a soft spreadable custard, not a pastry cream.
- Filling No. 3"Twice the rule" means double the boiled frosting above. Work quickly, since melted chocolate stiffens meringue within a minute or two, and warm the knife as directed.
- YieldThe single quantity fills and tops one 8- or 9-inch layer cake thinly, or frosts about a dozen cupcakes. It is best the day it is made; boiled frosting turns crusty and beads moisture by the second day.
Boiled icing was one American answer to the problem of frosting a cake before electric mixers and reliable confectioners' sugar were universal: hot syrup does the work that powdered sugar and butter do now, and a single egg white stretches a cup of sugar into enough covering for a whole cake. The technique is the same one confectioners call Italian meringue, and the cold-water ball test predates household candy thermometers by generations; the named stages, from thread through soft ball to crack, were standard kitchen vocabulary well into the twentieth century. Unsweetened chocolate came in a scored block, and recipes counted it in squares and cakes rather than ounces.