1. Whisk together egg, yolks, sugar, lemon zest and juice, and salt. Bring to a simmer in a medium saucepan over medium heat, whisking constantly. Cook until mixture is thickened, 4 to 5 minutes. Strain through a fine sieve into a bowl. Whisk in butter. Press plastic wrap directly onto surface of lemon curd, and refrigerate until cold, at least 1 hour or up to overnight.