Lemon Butter

This is lemon curd under an older name, cooked in an improvised double boiler. The method is sound; only the storage advice needs rethinking.

• TeacupA nineteenth-century tea-cup is smaller than a modern measuring cup — reckon it at about 3/4 cup of sugar, roughly 5 oz. A full cup gives a sweeter, less sharp curd.
• Butter the size of half an eggAbout 1 oz, or 2 tablespoons. Kitchen measures by egg-size were common where scales were not, and they are forgiving here.
• The pan set in a kettle of hot waterThis is a double boiler. Keep the water hot but not boiling and do not let the bowl touch it; direct or fierce heat scrambles the eggs into threads rather than thickening them.
• How thick, and when to stopStir constantly and pull it off when it coats the back of the spoon and holds a line drawn through it, around 170-175F. It sets further as it cools, so a curd that looks correct in the pan will be stiff in the jar.
• Rind before juiceGrate the zest into the sugar and rub it in before adding the juice — the sugar draws out the oils and the flavour is markedly stronger than zest added at the end. Take only the yellow, not the white pith.
• StrainingThe original does not mention it, but pressing the finished curd through a fine sieve removes any set egg white and the coarse rind, giving the smooth texture expected of a tart filling.
• StorageDisregard "kept for a long time in bottles or jars." An egg-and-butter curd is not a shelf-stable preserve: refrigerate it in a covered jar and use within a week or two, or freeze it.
• YieldOne large lemon and three eggs give roughly a cup and a half of curd -- enough to fill a batch of small tarts or a single 8-inch pastry shell.