Finely zest the lemons, taking only the yellow outer peel, then squeeze enough lemons to produce 1/2 cup of juice. Strain the juice to remove seeds and pulp.

In a medium, heavy-bottomed saucepan, whisk together the egg yolks, sugar, lemon zest, lemon juice, and salt until evenly combined.

Cook the mixture over medium-low heat, whisking constantly, for 8–12 minutes. Continue until it coats the back of a spoon and reaches approximately 170°F (77°C), but do not let it boil.

Remove the saucepan from the heat and whisk in the butter one piece at a time. Wait until each piece melts fully before adding the next, creating a smooth, glossy curd.

Pour the warm curd through a fine-mesh strainer into a clean bowl or jar. This removes the zest, any cooked egg particles, and small lumps.

Press plastic wrap directly against the surface of the curd to prevent a skin from forming. Cool it, then refrigerate for at least 1 hour, until thick and spreadable.