Lemon Filling
The grated rind and juice of 1 large lemon, 1 cup sugar, 2 eggs (or 4 yolks), 1 tablespoon butter; cook all together over boiling water until thick and use when cool.
This is lemon curd under an older name, meant for spreading between cake layers rather than jarring. The whole procedure hinges on gentle heat and patience over the double boiler.
- The hinge: indirect heat only"Over boiling water" means a double boiler or a heatproof bowl set over a pan of simmering water, not resting in it. Direct heat scrambles the eggs in seconds; over water the mixture thickens gradually and stays smooth.
- How thick is thickStir constantly and cook until the mixture coats the back of a spoon and holds a line drawn through it with a finger, roughly 8 to 12 minutes. On a thermometer that is about 170-175F; it will set considerably firmer as it cools.
- Whole eggs or yolksThe four-yolk version gives a richer, more opaque filling; two whole eggs make a lighter, paler curd. The recipe treats the two as interchangeable.
- Zest and strainGrate the rind finely on a microplane and remove no white pith. Straining the finished curd through a sieve catches any set egg and the coarser zest, which matters if you want a smooth layer.
- Sugar and lemon proportionsOne large lemon yields roughly 3 to 4 tablespoons juice and about a tablespoon of zest, which with a cup of sugar makes a sweet, mild filling by current tastes. Two lemons will give a sharper curd, with little effect on a set that depends mainly on the eggs.
- ButterA tablespoon is a small amount by modern curd standards. Whisk it in at the end, off the heat, for a glossier finish and a slightly softer texture.
- Yield and useAbout 1 to 1 1/4 cups, enough to fill one two-layer 8- or 9-inch cake or a small pie shell. Cool with plastic wrap pressed to the surface to prevent a skin, and refrigerate; it keeps about a week.
Lemon curd of this kind — citrus, sugar, eggs and a little butter thickened over water — appears in English and American cookery through the nineteenth century as a filling and preserve, and by the era of promotional flour-company cookbooks it was standard equipment in the cake chapter. Such booklets existed to sell flour, so the layer cakes get the detailed treatment and the fillings are compressed into a line or two on the assumption that any cook already knew what "cook over boiling water until thick" involved. The absence of cornstarch is worth noting: many commercial and later home lemon fillings lean on starch for body and economy, while this one relies entirely on egg protein, which is why the gentle heat is not optional.