Set up a double boiler – or use a bowl set over a pan of simmering water (make sure the bowl isn’t touching the water though) and place the lemon zest and juice, sugar and the butter into the bowl. Heat on a medium heat, stirring every few seconds until the butter melts and sugar dissolves. Just heat until it’s at this point – you don’t want to boil the mixture.
Mix together the eggs and egg yolk, then pour into the lemon mixture (still on the double boiler) whilst stirring with a whisk.
Continue to heat on a medium heat, stirring every few seconds with a whisk until the mixture thickens enough to coat the back of a spoon. This should take 5-8 minutes.
Once thickened, remove the bowl and leave to cool. Give it a stir every 5-10 minutes until the mixture is cool, then place into a sterilized jar. Screw on the lid and refrigerate until needed (or serve immediately!).
Notes
You may occasionally find that you get a few white lumps in your thickened lemon curd – this is due to the egg whites cooking at a slightly different temperate to the egg yolks. I usually leave them in there, but you can strain the lemon curd over a fine mesh sieve to remove if you prefer.