Add the water to the pot and place the bowl over the pot. Make sure the water doesn't touch the bottom of the bowl.
Turn the heat on to medium to high heat. The water should remain at a simmer, not a boil.
Place the butter into the double boiler (the bowl over the pot).
Once the butter has melted, add the sugar and lemon juice and stir until the sugar has dissolved.
Pour some of the mixture (I added 1 cup, then added another) into the well-beaten eggs. Whisk it as you add the hot butter, sugar and lemon juice.
Add the egg mixture into the bowl over the pot and keep whisking at all times.
Cook, stirring at all times until the mixture starts to thicken. This will take up to ten minutes, adjust the heat as needed. Add the pinch of salt (you can leave this out if you are using salted butter). The mixture is ready when it coats the back of the wooden spoon. It will also thicken on cooling.
Prepare jars in the dishwasher to clean and sterilize. The jars still need to be warm before filling them.
Pour the lemon curd through a sieve, this will help take any egg bits that may have occurred.
Pour the lemon curd into the hot jars and place the lids on.
Set the hot jars on some newspaper or a chopping board. Leave until they have cooled to room temperature.
Place into the fridge and use within a few weeks.