Combine yolks, lemon zest, lemon juice, and sugar in a small, heavy saucepan. Whisk to combine. Set over medium heat, and stir nearly constantly with a whisk, making sure to stir corners and bottom of pan. Cook until mixture is thick enough to coat back of spoon, leaving a distinct line that doesn’t fill in when swiped with a finger, 5 to 7 minutes.

Remove the saucepan from heat. Add butter, one piece at a time, stirring until the consistency is smooth. Transfer mixture to a small bowl. Lay a sheet of plastic wrap directly on the surface of the curd to avoid a skin from forming; wrap tightly. Let cool; refrigerate until firm and chilled, at least 1 hour.

Note: Either strain the egg yolks before combining with other ingredients or push the lemon curd through a strainer after the lemon curd is cooked to remove any of the chalazae, the white stringiness attached to the yolk and any bits of zest.

Makes about 1 cup.