Zest and juice the lemons. Finely zest 1–2 tablespoons from clean, dry lemons, avoiding the bitter white pith. Juice enough lemons to get about 1/2 cup fresh lemon juice. Strain the juice to remove seeds and pulp.

Whisk the base. In a medium, heavy-bottomed saucepan (off the heat), whisk together the eggs, powdered sweetener, lemon juice, lemon zest, and a pinch of salt until smooth. This helps prevent lumps later.

Cook low and slow. Place the pan over low to medium-low heat. Cook, whisking constantly, until the mixture thickens enough to coat the back of a spoon, about 7–12 minutes. Do not let it boil. If you see steam or tiny bubbles, lower the heat.

Add the butter. Remove from heat. Whisk in the butter, a few pieces at a time, until completely melted and silky. Add vanilla if using. The curd will look glossy and smooth.

Strain for ultra-smooth texture. For a professional finish, pour the warm curd through a fine-mesh sieve into a clean bowl or jar. This catches any bits of cooked egg or zest.

Cool and set. Press a piece of parchment or plastic wrap directly onto the surface to prevent a skin. Chill for at least 2 hours, or until thickened. It will continue to set as it cools.