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Lemon Filling
2 1/2 tablespoons flour 1/2 cup cold water 1 egg-yolk 1 cup sugar Juice and grated rind of 1 lemon 1 teaspoon butter Make a smooth paste of the flour and two tablespoons of the cold water. Cook the rest of the water, the sugar, grated lemon-rind and butter. When the sugar is dissolved and mixture boiling, stir in the flour mixture slowly. Cook until clear and smooth, about fifteen minutes. Add lemon-juice and beaten egg-yolk and cook two minutes. Cool before spreading on cake.
A flour-thickened lemon curd of the economical sort, using one yolk rather than the four or five a modern curd calls for. It comes together in a small saucepan in under twenty minutes.
- The hingeAdd the beaten yolk only after the starch has cooked out and the pan is off a hard boil, and stir constantly for the final two minutes. Dropped into a rolling mixture, the yolk scrambles and the filling turns grainy.
- Cook the flour fullyFifteen minutes sounds long by modern habit but is right here: flour needs sustained cooking to lose its raw taste, and the recipe's cue -- "until clear and smooth" -- is the real test. Use low heat and a heavy pan, and stir often, as this quantity scorches easily.
- Lemon juice lastAcid weakens starch thickening, so holding the juice to the end (as written) protects the set. Do not be tempted to add it with the rind.
- RindGrate only the yellow zest; the white pith is bitter. A medium lemon gives roughly a tablespoon of zest and 2 to 3 tablespoons of juice, so if yours is small you may want a second lemon, bearing in mind that extra acid loosens the set.
- CoolingPress a piece of parchment or plastic directly on the surface while it cools, or the top skins over and streaks the cake. Cooled to room temperature it spreads; chilled it stiffens considerably.
- YieldAbout one cup -- enough to fill between two 8- or 9-inch layers, with nothing to spare. Double it if you want a thick filling or a three-layer cake.
- KeepingEgg and no preservative: refrigerate and use within three days. Fill the cake the day it is served, as the filling softens crumb over time.
Flour-thickened lemon fillings like this one belong to a long American tradition of stretching expensive eggs with cheap starch. A proper lemon curd is yolks, butter, sugar and juice cooked to a custard, but that meant several eggs and careful heat control; a home cook building a layer cake could get a bright, glossy filling from a single yolk and two and a half tablespoons of flour, and the flour also made the result sturdier between layers than curd, which tends to seep. The New Delineator Recipes was published by Butterick, which also published the Delineator magazine. The same starch-thickened technique underlies the lemon meringue pie fillings of the same era.