Prepare an ice bath fitted with a medium bowl; set aside. Whisk together yolks, zest, juice, and sugar in a small saucepan. Set over medium heat, and stir constantly with a wooden spoon. Cook until mixture is thick enough to coat the back of the wooden spoon, 5 to 7 minutes.
Remove pan from heat. Add butter, 1 piece at a time, stirring until incorporated. Pass through a fine mesh sieve into prepared medium bowl. Stirring frequently, let stand until cool.
Place a sheet of plastic wrap directly on surface of curd to prevent skin from forming; wrap tightly. Refrigerate until firm and chilled, at least 1 hour.