Chocolate Frosting (II)

A cooked chocolate frosting brought to the soft-ball stage, closer to a fudge icing than a buttercream. The whole recipe hinges on the cold-water test, so read that point first.

• The cold-water test"Candy when dropped in cold water" is not precise, but in a frosting of this kind it most likely means the soft-ball stage, around 235-240F on a modern thermometer: a spoonful dripped into cold water forms a ball that holds its shape but flattens when lifted out. Much below that the frosting stays a sauce; much above it and it is liable to set hard and grainy on the cake.
• Grated chocolateThis most likely means a plain block of unsweetened chocolate, grated rather than chopped. Half a cup grated is very roughly 1.5 to 2 oz; unsweetened baking chocolate, chopped fine, is the nearest equivalent. Cocoa powder is not a direct substitute, as it behaves differently in a boiled syrup.
• Sweet milkSimply fresh milk, specified to distinguish it from the sour or buttermilk that was equally common in a period kitchen. Whole milk gives the best body.
• "Small cup half full"Genuinely ambiguous. A small cup of the era might hold anything from a scant half-cup to a teacup's worth, so this is somewhere around 1/4 cup of milk. Start at the low end; a wetter mixture only means longer boiling.
• The egg yolkWhisk the yolk into the milk before the pan goes on the heat rather than adding it to a hot mixture, or it will scramble in threads. The syrup passes well above 160F, so the yolk is fully cooked by the time the test is met.
• Pan and stirringUse a heavy-bottomed saucepan and stir the whole time. Milk and grated chocolate scorch readily, and burnt chocolate cannot be rescued.
• Getting it onto the cakeTake the pan off the heat and let it cool a few minutes, then beat until it thickens and loses its gloss slightly. Spread quickly; it firms as it cools.
• YieldProbably about a cupful, roughly enough to cover the top of one 8- or 9-inch layer. Double it for generous coverage of a two-layer cake.