In a medium saucepan, whisk together lemon juice, lemon zest, sugar, eggs, and salt until smooth and lightened.

Cook over medium‑low heat, whisking constantly, until mixture thickens and begins to bubble or coats the back of a spoon (about 6–9 minutes).

Remove from heat and strain through a fine sieve into a clean bowl to remove any solids.

Stir in butter pieces (and vanilla, if using) until fully melted and incorporated.

Press plastic wrap directly onto the surface to prevent a skin from forming. Let cool to room temperature, then refrigerate until set, at least 1 hour. Keeps about one week chilled.