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Chocolate Frosting
One cupful grated chocolate, one cupful sugar, one tablespoonful corn starch, one wine glass water, and boil sufficiently thick.
A four-ingredient cooked frosting from the era before confectioners' sugar was standard in home baking. The instructions assume you know what "sufficiently thick" looks like, so the notes below are mostly about that.
- A wine glassIn American cookbooks of this period a wine glass is generally reckoned at 2 fluid ounces, or a quarter cup. Conventions vary from book to book; if your mixture stalls out too stiff, a spoonful more water is the fix.
- Grated chocolateThis means unsweetened block chocolate grated on a box grater, not cocoa powder and not chips. A cup of loosely grated chocolate is roughly 3 to 4 oz by weight; chopping 4 oz of unsweetened baking chocolate fine gets you to the same place.
- Slurry the cornstarch firstWhisk the tablespoon of cornstarch into the cold water before it meets anything hot. Added dry to a warm pan it clumps, and you cannot whisk the lumps out of a mixture this thick.
- What "sufficiently thick" meansCook over low to medium heat, stirring constantly, until the mixture coats the back of the spoon and a track drawn through it holds for a second or two. That is about 3 to 5 minutes after it comes to a simmer. Cornstarch needs to actually boil to reach full thickening power, but past a minute or so of hard boiling it begins to thin again.
- ScorchingSugar and chocolate at a boil catch on the bottom of a thin pan fast. Use a heavy-bottomed saucepan and keep the spoon moving across the whole base, or set the pan over simmering water and accept a longer cook.
- Spread warmThis sets as it cools, so pour it over the cake while it is still pourable and coax it out with an offset spatula. Reheat gently with a teaspoon of water if it stiffens in the pan.
- YieldEnough to cover the top and sides of a single 8- or 9-inch layer, or the top of a sheet cake. Double it for a two-layer cake with filling.
Cooked chocolate frostings like this one belong to the decades when the alternative was a raw icing of egg white beaten with sugar. Block chocolate was sold in hard unsweetened cakes to be grated as needed, which is why so many nineteenth-century recipes begin with "grated chocolate" rather than a weight; cocoa powder and boxed confectioners' sugar were both available by the 1880s. The cornstarch is doing the work an egg or a long sugar boil would otherwise do, thickening the syrup enough to hold on the side of a cake, and cornstarch was by then being pressed into service for puddings, blancmanges and sauces of every kind. The "wine glass" measure is a reminder that these books were written for cooks measuring with whatever stood on the sideboard; graduated cups were only beginning to be standardized in American cookery.