1-In a small saucepan, stir together lemon: In a small saucepan, stir together lemon zest, lemon juice, sugar, and salt. Set over medium heat and cook for about 5 minutes, or until the sugar and salt have dissolved.
2-Meanwhile, in a medium bowl beat together: Meanwhile, in a medium bowl beat together 3 eggs and 4 egg yolks. Whisk it very well.
3-When the liquid on the stove is: When the liquid on the stove is ready, slowly pour some of it into the bowl with the eggs, whisking constantly.
4-Return the pot to the stove and: Return the pot to the stove and add the rest of the eggs while stirring.
5-Continue to whisk pretty constantly over medium: Continue to whisk pretty constantly over medium heat for about 5 minutes, or until the mixture has thickened. When you drag your whisk over the top of the mixture, it should leave marks.
6-Pour the curd through a mesh strainer: Pour the curd through a mesh strainer into a new bowl (or the same one, washed).
7-Stir the chopped butter into the lemon: Stir the chopped butter into the lemon curd until it’s completely melted.
8-Press a square of plastic wrap directly: Press a square of plastic wrap directly on top of the lemon curd.
9-Refrigerate until cold, about 2-4 hours: Refrigerate until cold, about 2-4 hours.
10-Transfer to a tupperware: Transfer to a tupperware. Keeps for 1-2 weeks.