1. Place the sugar in your food processor.
2. Add the lemon zest and process until well combined.
3. Add the butter and process to blend.
4. With the processor running, add the egg yolks one at a time, followed by the lemon juice and the salt.
5. Pour the mixture into a small saucepan and cook very slowly over a low heat, stirring constantly, until slightly thickened (about 10 minutes). Do not allow the custard to boil. To check the consistency, lift your stirring spoon from the mixture and swipe a finger across its back; if your finger leaves a clear path on the spoon, the curd is ready to come off the heat.
6. Immediately remove from the heat and pour the curd through a strainer into a medium bowl.
7. Set aside to cool to room temperature.
8. Refrigerate for 4–6 hours, until well chilled. Curd keeps in the refrigerator for several weeks.