Zest two Meyer lemons finely; then juice all lemons until you have about 1/2 cup (120 ml) of juice. Strain to remove pulp and seeds.

In a medium bowl, whisk together 3 egg yolks, 1 whole egg (if using), and 3/4 cup sugar until the mixture is pale and slightly thickened.

Place a saucepan with 1–2 inches of simmering water over medium-low heat. Put the egg-sugar bowl over the saucepan (double boiler) or use the same saucepan on the lowest heat, ensuring gentle cooking.

Slowly whisk in the warm lemon juice to the egg mixture in a thin stream to temper the eggs. Continue whisking constantly.

Cook, stirring or whisking frequently, until the mixture thickens and coats the back of a spoon — about 8–12 minutes. The curd should reach about 170–175°F (77–80°C) if you use an instant-read thermometer.

Remove from heat. Whisk in the cubed butter a few pieces at a time until fully incorporated and silky. Add a pinch of salt and taste; adjust with a little extra lemon juice or sugar if needed.

Pour the curd through a fine-mesh sieve into a clean jar to remove any zest bits or cooked egg bits for an ultra-smooth texture.

Cool to room temperature, then refrigerate. The curd will thicken further as it chills.