In a medium saucepan, combine sugar, eggs, lemon juice, and zest.
Whisk the mixture until well combined.
Place the saucepan over medium heat.
Cook the mixture, stirring constantly, until it thickens (about 10 minutes).
Remove from heat and strain through a fine-mesh sieve.
Stir in the cubed butter until melted and smooth.
Add a pinch of salt and mix well.
Transfer the curd to a clean jar or container.
Allow to cool at room temperature for about 30 minutes.
Refrigerate for at least 2 hours before serving.