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Lemon Butter for Roll Cake
Dissolve one cupful of sugar in the juice of one large lemon, beat three eggs very light, add and mix well, add a piece of butter the size of a walnut, cook in a farina kettle, stirring constantly until pretty thick.
This is a lemon curd by another name, thinned enough to spread on a sponge sheet before rolling. The whole recipe hinges on gentle indirect heat and constant stirring.
- Farina kettleA farina kettle is a double boiler — an inner pan set over a lower pan of simmering water. Use any heatproof bowl over a saucepan of barely simmering water, with the water not touching the bowl.
- Butter the size of a walnutRoughly 1 to 2 tablespoons; period sources vary. Add it in one piece as directed; it melts in as the curd thickens and helps the finished spread set smoothly.
- One large lemonAbout 3 tablespoons of juice for a full cup of sugar, which makes a sweet, mild curd. If your lemons are small, use two and expect a sharper result. Grated zest is not called for but adds considerable flavour.
- Whole eggs, not yolksThree whole eggs give a lighter, paler curd than a yolk-only version, and the whites are what make it thicken relatively quickly. Beat them thoroughly before they meet the sugared juice so no strands of white cook out.
- Judging "pretty thick"Stir until the mixture coats the spoon and holds a line drawn through it, around 170–175F. Off the heat it firms further as it cools, so stop while it still moves.
- Strain itPour the finished curd through a fine sieve while hot. This removes any set egg and the chalazae, and is the difference between a glossy filling and a grainy one.
- Yield and useMakes roughly a cup and a half — enough to fill one jelly-roll sponge. Cool to room temperature before spreading, or it will soak the cake and tear it when rolled.
- KeepingRefrigerate in a covered jar and use within a week. It is an egg custard, not a preserve, despite the sugar.
Lemon butter, lemon cheese and lemon curd are all names for the same thing: eggs, sugar, butter and citrus thickened over water rather than direct heat. "Butter" and "cheese" reflect an older sense of the words, meaning a thick spreadable mass rather than a dairy product — the same usage that gives us apple butter and damson cheese. It is a natural filling for a rolled sponge cake: a thin sheet cake baked quickly, spread while barely cool, and rolled in a cloth. The direction to use a farina kettle is suggestive in itself — the vessel was sold for cooking breakfast cereals, and it seems cooks pressed it into service as the household double boiler for custards, sauces and chocolate.