Lemon Jelly Cake (Bake sponge-cake batter)

A layer cake with a cooked lemon curd between the layers. The filling is the whole recipe; the cake itself is borrowed from the sponge-cake entry elsewhere in the book.

• Jelly-cake pansThese are shallow round layer pans, an inch or so deep. The recipe does not say how many layers to divide the batter into; thin layers bake in a fraction of the time a single deep pan would need.
• Butter size of an eggAbout 2 ounces, or 4 tablespoons. The measure is common in nineteenth-century American books, though period equivalents for it varied.
• A slow fireLow heat, and in practice a bowl over simmering water is safer than direct contact with the pan. This is a lemon curd, and it will scramble at the edges before the centre thickens if the heat is brisk.
• The hinge: stop at the thickening, not the boilThe instruction to cook "until it boils" is a doneness cue rather than an instruction to boil hard. Curd thickens at about 170 to 175F, when it coats a spoon and holds a line drawn through it; the heavy dose of sugar here raises the temperature at which the eggs set, but a hard boil still risks curdling. Strain through a sieve if it looks grainy.
• Sugar quantityTwo cupfuls to three eggs and two lemons makes a very sweet, quite firm filling by modern taste. Cutting the sugar while keeping the same lemon is possible, but it changes how the curd sets.
• Grated rindTake only the yellow zest, not the white pith beneath. Adding it at the end rather than during cooking keeps the flavour brighter, though the original method works.
• Assembly and yieldSomewhere between two and three cups of filling. Let the curd cool to spreadable before it goes between the layers, or it will run off the edges. The finished cake keeps two or three days refrigerated, the curd being egg-based.
• Lemon icingProbably a simple glaze of sifted powdered sugar slaked with lemon juice rather than a buttercream. Sifted sugar alone, as the alternative allows, suits the cake perfectly well.