In a medium bowl, whisk together the eggs, lemon juice, zest, and sugar until smooth.
Pour the mixture into a medium saucepan and set it over medium-low heat.
Stir constantly with a rubber spatula, scraping the sides and bottom, until the curd thickens and coats the back of the spoon—about 10 minutes.
Remove from heat and whisk in the butter, one piece at a time, until fully melted and smooth.
Strain the curd through a fine mesh sieve into a clean bowl for extra silkiness.
Let it cool, then transfer to a jar and refrigerate. It’ll thicken more as it chills.