Method

Make a bain-marie with a heatproof bowl (glass in best) & a large saucepan, try to find a bowl which goes down into the pan by about a 3rd the way down, or use a double boiler if you have one.

In said bowl whisk the yolk, salt & sugar together, then add lemon juice & zest & whisk together, place the pan on a med heat with water not deep enough to touch the bottom of the bowl but deep enough not to boil away, then boiling place the bowl with mixture on top of the pan & whisk until it thicken & coats the back of a spoon (if it gets to hot you will have sweet lemon scrambled eggs), if you have a thermometer it should be at 75'c, remove from the heat.

Straightaway add the butter in roughly 20g amount whisking in the mixture, keep going until all the butter is mixed in.

Pour curd into sterile jar/s & lid/s, allow to cool & store in the fridge for up to 10 days.