Directions

1. Combine yolks, lemon zest, lemon juice, and sugar in a small saucepan. Whisk to combine. Set over medium heat, and stir 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, 5 to 7 minutes.
2. Remove saucepan from heat. Add the butter, one piece at a time, stirring with the wooden spoon until consistency is smooth.
3. Transfer mixture to a medium 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. Store, refrigerated in an airtight container, up to 2 days.