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Lemon Filling
1 cup sugar 2 1/2 tablespoons flour Grated rind 2 lemons 1/4 cup lemon juice 1 egg 1 teaspoon butter Mix sugar and flour, add grated rind, lemon juice, and egg slightly beaten. Put butter in saucepan; when melted, add mixture, and stir constantly until boiling-point is reached. Care must be taken that mixture does not adhere to bottom of saucepan. Cool before spreading.
A flour-thickened lemon filling, sturdier and less rich than a butter-heavy curd, meant to be spread cold between cake layers. It comes together in five minutes over moderate heat.
- Bring it fully to the boilFlour needs to reach a boil to lose its raw taste and set properly, and the single egg is cooked in the process. The recipe stops at boiling point; if you want to be sure of the flour, a few seconds at a bubble, stirring, will do it.
- Heat and stirringUse medium-low heat and a heavy saucepan, and stir into the corners with a flat-ended wooden spoon or silicone spatula rather than a whisk. The warning about the mixture adhering to the bottom is the hinge of the recipe: once it catches, the scorched flavour goes right through.
- The butter is a coating, not an enrichmentThe butter is barely an enrichment -- One teaspoon melted in the pan first reads as a way to keep the mixture from sticking rather than to enrich it, though the recipe does not say so. If you want a glossier, richer filling, stir an additional tablespoon or two of cold butter in off the heat at the end.
- ZestThe rind of two lemons is a large amount of oil for one cup of sugar, which is the point. Take only the yellow, and rubbing the zest into the sugar with your fingertips before adding anything else draws out more of it.
- Juice quantityA quarter cup is roughly the juice of one and a half medium lemons, so two lemons will give you both the zest and the juice with a little to spare.
- Consistency and coolingIt thickens noticeably as it cools, so stop while it still falls thickly from the spoon. Press plastic wrap directly onto the surface to prevent a skin, and chill until cold and spreadable.
- Yield and keepingAbout one cup, enough to fill one 8- or 9-inch two-layer cake. Refrigerate and use within three days.
Flour-bound lemon fillings like this one belong to the cooking-school tradition that dominated American household books around 1900, when precise level measurements and a short list of reliable, cheap thickeners were the selling point. Lemon curd proper, an English preserve, leans on egg yolks and a good deal of butter; this version substitutes two and a half tablespoons of flour and a single whole egg, cutting the cost and producing a filling firm enough to sit between cake layers. A thinned version of much the same mixture, poured into a shell under meringue, is close to the lemon pie fillings of the period, though those more often used cornstarch.