Wash and dry the lemon. Zest them first, then halve them and juice them.
Place the diced butter, sugar, salt, lemon zest, and juice into a heatproof bowl.
Bring a pan of water to a gentle simmer. Place the bowl on top; its bottom should not touch the water.
Heat the ingredients, stirring often, until the sugar dissolves.
Eggs: In the meantime, beat the eggs and strain them into another small bowl.
Add them to the sugar mixture and stir continuously until it thickens slightly and becomes opaque. You should be able to trace a path with your finger on the curd on the spoon (See picture in the blog post).
Transfer the curd to a clean bowl, place a piece of clingfilm directly onto the surface of the curd and let cool completely.