Finely grate the zest from 3 unwaxed lemons, taking only the yellow outer skin and avoiding the bitter white pith. Juice the lemons until you have 120 ml, then strain the juice to remove seeds.

In a heatproof bowl or saucepan, whisk together the egg yolks, whole egg, sugar, lemon zest, lemon juice, and salt until evenly combined.

Cook the mixture over low to medium-low heat, stirring constantly with a whisk or silicone spatula. Continue for 8 to 15 minutes, until it coats the back of a spoon and reaches approximately 170 to 175°F (77 to 80°C); do not let it boil.

Remove the curd from the heat and whisk in the butter one piece at a time. Keep whisking until each piece melts and the mixture becomes smooth, glossy, and richly golden.

Strain the warm curd through a fine-mesh sieve into a clean container to remove zest, cooked egg particles, or small lumps. Press plastic wrap directly onto the surface and chill for at least 30 minutes, or until set.