In a glass bowl add in the eggs, lemon juice and zest, sugar and salt (if using unsalted butter.)
Bring a saucepan of water to boil, then turn the heat down so the water is simmering. Place the glass bowl over top.
Using a whisk, whisk all the ingredients together and keep whisking for around 12-15 minutes until the curd starts to thicken.It will only thicken slightly. Check to see if it is thick enough by dipping in a spoon, running your finger through the curd on the back of the spoon and seeing if it draws a line though the curd, that stays there.
Once thick enough, take off the heat and whisk in the butter cubes.
Pour through a sieve into a clean jar. Store in the fridge in an airtight container for up to two weeks or freeze it for up to three months.