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Caramel Frosting
1 cup granulated sugar 1/3 cup Karo, Blue or Orange Label 1 egg white 2. tablespoons water Few grains salt 1/4 teaspoon vanilla Combine the Karo, sugar, water and salt and boil until it threads. Add the vanilla and pour gradually into the egg white . Add the vanilla and pour gradually into the egg white which has been whipped stiff. Beat until thick, then set over hot water and cook about ten minutes, till firm around the edge.
Beat till the mixture will hold its shape and spread on the cake.
A boiled meringue frosting of the seven-minute type, stabilised with corn syrup so it stays glossy rather than turning grainy. The printed text repeats a line — "Add the vanilla and pour gradually into the egg white" — which is a compositor's error, not a second addition.
- The hinge: pour slowlyThe hot syrup must go into the stiff whites in a thin, steady stream at the side of the bowl while the beater runs. Dumped in, it scrambles the whites and the frosting will never hold air.
- "Boil until it threads"The thread stage is 230-235F on a candy thermometer — a syrup that trails a fine thread from a lifted spoon. Below that the frosting stays soupy; much above it and you are heading for a hard, brittle icing.
- Karo labelsKaro was sold in colour-coded grades of differing depth, the blue-labelled syrup being one of the lighter ones; darker grades carried more colour and a stronger flavour, which may be why the recipe is called caramel. Light corn syrup is the straightforward substitute; dark corn syrup will give a deeper colour if that is what the Orange Label was.
- Not actually caramelNothing here is caramelised. The colour and flavour come from the syrup itself, so a frosting made with light corn syrup will be white and taste of vanilla — perfectly good, but not what the title promises.
- Over hot water, ten minutesThe double-boiler stage is what sets the meringue and brings the egg white through a cooked temperature. Keep the water hot but not at a rolling boil, and beat continuously; the frosting is done when it is thick, opaque and firm at the bowl's edge.
- One egg whiteA large modern egg white is about 2 tablespoons, close enough to what the book assumed. If the whites are small, use one and a half rather than two, or the syrup will not sweeten and stabilise them.
- Weather mattersBoiled frostings absorb moisture. On a humid day take the syrup to the top of the thread range and expect a softer set; the frosting will also weep if the cake is refrigerated.
- Yield and handlingRoughly enough to cover a single 8- or 9-inch layer, or to top a square cake. Spread it while still warm and pliable; it crusts as it cools and tears if worked cold.
Boiled frostings like this one belong to the era of the rotary eggbeater, when cooks worked by the thread test rather than a thermometer and had learned that replacing part of the sugar in a syrup with glucose stopped crystals forming. Commercial corn syrup, sold under the Karo name from the early 1900s, made that trick available in a bottle, and the maker promoted it through free and cheap recipe booklets of exactly this kind -- every recipe naming the brand, often down to the label colour. Boiled or double-boiler frostings suited such a campaign well: dramatic on a plain layer cake, and reliant on the one ingredient the booklet existed to sell.