Chocolate Filling

A cooked chocolate cream thickened with a single yolk, meant to be spread between cake layers rather than poured over them. It comes together in under ten minutes and stiffens further as it cools.

• "1/4 cake chocolate"Nineteenth-century chocolate was sold in pressed cakes of varying size, so this quantity is genuinely ambiguous: a quarter of a one-ounce square gives almost no flavour, a quarter of a half-pound cake gives two ounces. The proportions here (a cup of sugar, half a cup of milk) suit roughly 1 to 2 oz of unsweetened chocolate.
• Which chocolateBaking chocolate of this period was usually unsweetened, which is the likeliest reason the sugar runs so high. Substituting a modern semi-sweet or milk bar will make the filling cloying; use unsweetened baking chocolate and keep the sugar as written.
• Grating still mattersGrating was the standard way to get a hard cake of chocolate to dissolve evenly. Chopping fine or shaving with a peeler does the same job; large chunks will sit at the bottom of the pan and scorch.
• "Boil till thickened"This is closer to a sugar-syrup stage than a sauce. Stir constantly over medium heat and boil until the mixture coats the spoon and leaves a brief trail when the spoon is drawn through it.
• Adding the yolkTake the pan off the heat, stir a spoonful or two of the hot mixture into the beaten yolk, then return the lot to the pan before the two minutes of cooking. Dropping a cold yolk straight into boiling syrup gives you specks of cooked egg.
• Vanilla lastThe instruction to add extract off the fire is deliberate. Alcohol-based extract loses most of its aroma in a boiling pan.
• Yield and settingRoughly a cupful, enough to fill one two-layer cake. It is loose when hot and spreadable when cool; let it stand until it holds a soft mound before using, and thin with a teaspoon of milk if it goes past that.