Whisk the egg yolks and sugar in a heavy bottomed pan until pale and frothy.

Add the lemon juice and zest.

Carefully, with a sharp knife, split the vanilla pod in half.

Scrape the paste from the inside and add to the egg mixture.

Add the pod and a small pinch of salt as well.

Heat the mixture over medium heat, stirring constantly until it has thickened and coats the back of a spoon with a thick rich layer.

Remove from heat and whisk the butter in, one tablespoon at a time until it is has melted smoothly into the curd.

Allow to sit for 20 minutes before removing the vanilla pod.

Cover, with plastic wrap directly touching the surface, and chill until set, or at least one hour.