In a large stock pot, add sugar, lemon zest, and lemon juice. Mix well.

In a separate bowl add the eggs and salt and whisk them until frothy. Add the egg mixture to the sugar mixture. Mix well. Heat the mixture over low heat, stirring constantly, and allow the curd to thicken to the consistency of a warm custard. When it reaches 170°F, turn off the heat.

Strain the lemon curd over a sieve into a large bowl. Stir in the butter until completely melted. Pour into an airtight container and store in the refrigerator for about 3 weeks.

You can freeze your lemon curd in freezer bags for up to a year.