How to Make It
In a large heatproof bowl, whisk the egg yolks with the sugar, lemon zest and juice. Set the bowl over a medium saucepan of simmering water and cook, stirring constantly with a wooden spoon, until thickened, 6 to 8 minutes. Remove from the heat and stir in the butter, one piece at a time, until incorporated. Place a piece of plastic wrap on the surface of the curd and refrigerate until chilled, about 30 minutes.