Whisk 1 cup sugar, cornstarch, salt and water in a saucepan and cook over medium heat until clear, thick and bubbling.
Whisk the egg yolks, temper them with 1 cup hot mixture and return everything to the pan; cook for 2 minutes, then stir in the lemon juice, zest and butter.
Pour the piping-hot filling into the baked crust and immediately beat the egg whites with the cream of tartar to soft peaks.
Add the remaining sugar gradually and beat to glossy stiff peaks, then spread the meringue over the hot filling all the way to the crust edges.
Bake at 350°F for 14 to 16 minutes until browned, cool for 1 hour and refrigerate for at least 4 hours.