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Chocolate Icing
11/2 squares chocolate 2 tablespoons Karo Red Label 1/2 teaspoon vanilla Few grains salt Sifted confectioner's sugar to spread about 1½ cups Melt the chocolate, add the Karo, salt and vanilla and gradually the confectioner's sugar, beating with a wire whisk.
A three-minute glaze rather than a whipped buttercream, and one that depends entirely on the corn syrup for gloss and flexibility. A few of its terms have shifted since it was written.
- Squares of chocolateA square is one ounce of unsweetened baking chocolate, so this calls for 1 1/2 oz. Modern bars are often scored into half-ounce pieces, so weigh rather than count.
- Karo Red LabelThat was Karo's dark corn syrup; the Blue Label was light. Dark corn syrup, with its trace of refiners' syrup and slightly molasses-ish edge, is the closest match. Light corn syrup works and gives a cleaner chocolate flavour.
- Why the syrup is thereCorn syrup is a glucose syrup, not an invert sugar, but like invert sugar it interferes with crystallisation and keeps the icing from setting hard and grainy. Without it the glaze is more likely to set brittle and crack when the cake is cut.
- Melting the chocolateUse a bowl over barely simmering water or short bursts in a microwave. Direct heat will scorch 1 1/2 oz of chocolate almost instantly, and any stray water will seize it.
- "Sugar to spread"The 1 1/2 cups is a target, not a rule. Add it a few tablespoons at a time until the icing falls from the whisk in a thick ribbon; if it stiffens past spreading, a teaspoon of hot water or cream brings it back.
- Sift, genuinelyConfectioners' sugar packs into hard lumps that a whisk will not break up in a mixture this stiff. Unsifted sugar shows as white specks in the finished glaze.
- Yield and useProbably enough to cover the top of a single 8- or 9-inch layer, or roughly a dozen cupcakes, though the recipe gives no yield. Spread while still warm; it thickens quickly as the chocolate sets.
Recipe booklets like this one were produced in quantity by American food manufacturers in the first decades of the twentieth century, and the brand-name call for "Karo Red Label" rather than simply "corn syrup" suggests promotional literature from the Corn Products Refining Company or one of its licensees. Karo, launched in the early 1900s, was sold hard to home cooks as a substitute for and improvement on cane sugar syrups, and confectionery was the strongest argument for it: candy makers had long known that a glucose or invert syrup keeps icings, fudges and fondants from turning grainy.