1. Combine yolks, lime zest, lime juice, and sugar in a saucepan. Whisk. Set over medium heat; stir constantly with a wooden spoon, making sure to stir sides and bottom of pan. Cook until thick enough to coat back of wooden spoon, 5 to 7 minutes.

2. Remove saucepan from heat. Strain curd into a bowl. Add butter, a few pieces at a time; stir with wooden spoon until smooth.

3. Lay plastic wrap directly on the curd's surface to keep 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.