Whisk the yolks, sugar and zest in a medium pot.

Whisk in the lemon juice.

Cook over medium low whisking until thick enough to coat the back of a spoon.

Remove from the heat.

Stir in the diced butter.

Add vanilla and salt.

Pour through a fine mesh strainer.

Serve warm immediately or allow to cool with cling wrap pressed against the surface of the curd to prevent a skin from forming.

Refrigerate.

Easily doubled.