This recipe makes a nice thick curd for filling layer cakes.
Pass the egg yolks through a strainer to remove any traces of white. Combine the yolks, lemon zest, lemon juice, and sugar in a heavy saucepan. Whisk the mixture to combine, then switch to a wooden spoon. Stir constantly over medium heat, making sure to stir all the sides and edges of the saucepan for even cooking. Cook until the mixture is thick enough to coat the back of a spoon, 20 to 25 minutes.
Remove the saucepan from heat. Add the butter, one piece at a time, stirring with a wooden spoon to incorporate into a smooth mixture. Transfer to a medium bowl, cover with plastic wrap, and place in the refrigerator until firm and chilled, at least one hour. May be made one day ahead.