In a medium bowl, whisk together the eggs, granulated sugar, cornstarch (optional) fresh lemon juice, and lemon zest.

Cook the mixture over low to medium heat, either in a double boiler (by placing the bowl over a saucepan of simmering water) or directly in a saucepan. Keep whisking constantly to prevent the eggs from curdling.

Continue to cook, stirring nonstop, for about 15–20 minutes, until the lemon curd thickens and coats the back of a spoon.

Remove from the heat and slowly whisk in the softened butter, a few pieces at a time, until fully melted and the curd is smooth and glossy.

Strain the lemon curd through a fine-mesh sieve to remove any bits of cooked egg and zest.

Transfer the lemon curd to a clean jar or bowl, cover with a lid or press plastic wrap directly onto the surface, and refrigerate until completely cooled.

Once chilled, use it to fill cakes, tarts, cupcakes, or simply enjoy it by the spoonful!