1. Heat lemon juice in small nonreactive saucepan over medium heat until hot but not boiling. Whisk eggs and yolk in medium nonreactive bowl; gradually whisk in sugar. Whisking constantly, slowly pour hot lemon juice into eggs, then return mixture to saucepan and cook over medium heat, stirring constantly with wooden spoon, until mixture registers 170 degrees on instant-read thermometer and is thick enough to cling to spoon, about 3 minutes (see photos, below).

2. Immediately remove pan from heat and stir in cold butter until incorporated; stir in cream, vanilla, and salt, then pour curd through fine-mesh strainer into small nonreactive bowl. Cover surface of curd directly with plastic wrap; refrigerate until needed.