Combine yolks, lemon zest, lemon juice, and sugar in a small saucepan. Whisk to combine.
Cook over low to medium heat, stirring constantly with a wooden spoon, making sure to stir sides and bottom of pan. Cook until mixture is thick enough to coat back of wooden spoon, about 5 minutes.
Remove saucepan from heat. Add butter, one piece at a time, stirring with the wooden spoon until consistency is thick and smooth.
Transfer mixture to a bowl. Lay a sheet of plastic wrap directly on the surface of the curd to avoid a skin from forming; wrap tightly.
Refrigerate until firm and chilled, at least 1 hour.