Lemon Filling
1 cup sugar Yolks of 2 eggs 1/2 tablespoon butter Juice and grated rind of 1 lemon Put all the ingredients in a double boiler and stir and cook until the mixture thickens. Spread between the layers of cake and ice the top with a white icing made from the whites of eggs not used in the filling.
This is a lemon curd by another name, thin enough to spread between cake layers. It is a short recipe with one real hazard: heat.
- Keep it below a simmerThe water in the lower pan should steam, not boil, and the bowl should not touch it. Yolks set around 160-170F; past that they scramble and the filling turns grainy no matter how hard you stir.
- Stir constantly, not occasionallyWith no starch or flour in the mix, the yolks alone do the thickening and they set fastest where they touch hot metal. A silicone spatula worked into the corners beats a spoon.
- How thick is thickTake it off when it coats the back of a spoon and holds a line drawn with a finger, roughly 8 to 12 minutes. It firms as it cools, so a filling that looks right in the pan will be noticeably stiffer cold.
- The lemonFigure about 3 tablespoons of juice; if yours yields much more, the filling will be looser and sharper.
- Zest before juicingGrate only the yellow, and add the rind at the start so its oil perfumes the sugar. If you dislike the texture, strain the finished filling while hot.
- ButterHalf a tablespoon is a token amount by modern curd standards. Stirring it in off the heat at the end can give a glossier, smoother result than cooking it through, though the recipe as written adds it at the start.
- The white icingThe book assumes you will use the two leftover whites. Choose a boiled or Italian-meringue icing, in which hot syrup cooks the whites, rather than an uncooked white-and-sugar glaze.
- YieldAbout a cup, enough for one layer of an 8- or 9-inch cake. Double it for a three-layer cake. Chill the filled cake briefly before icing so the layers do not slide.
Lemon curd, called lemon cheese in older English books, began as a preserve spooned onto bread and only later became a standard cake filling. The name is often said to reflect an early method in which cream was curdled with lemon juice and the solids drained, though that derivation is uncertain; by the nineteenth century the recipe had settled into the egg, sugar, butter and lemon version given here. The filling here ends with an instruction about what to do with the spare whites rather than a recipe for the cake itself. Nothing is wasted, and the two halves of the egg are accounted for on the same page.