Stir together lemon zest, lemon juice, sugar, and salt. Cook for 5 min, until dissolved Whisk together 3 eggs + 4 egg yolks. Slowly pour some of this liquid into the bowl with the eggs, whisking to temper. Add eggs while stirring . Whisk over medium heat for about 5 minutes, or until the mixture thickens. Strain out solids using mesh strainer. Stir chopped butter into curd until melted. Plastic wrap directly ON TOP of lemon curd. Refrigerate 2-4 hours.