Zest lemons, being careful not to remove the pith (white part under the skin). The white pith tends to make the curd bitter. Either chop the zest or place it in a food processor. Add the sugar, lemon juice and salt and pulse until combined.
Cream the butter then beat in the sugar mixture. Add the eggs 1 at a time until combined.
Pour the mixture into a saucepan and heat over low heat until it starts to thicken. This may take about 10–15 minutes. Stir continuously. Just before the pot starts to simmer, you will see the curd start to thicken. Make sure you don’t let it boil. Remove from heat and cool. Package it up into jars and refrigerate.