Blind bake the pie crust, and allow to cool completely.
Make the meringue: In a small bowl, combine the sugar with cornstarch and set aside. In a bowl of a standing mixer, whip the egg whites with cream of tartar for 1 minute or until soft peaks. Then slowly add in the sugar mixture (1 tablespoon at a time) until stiff peaks form. Right before stiff peaks form make sure to add in the vanilla extract.
Make the lemon curd filling: In a medium bowl, whisk the eggs very well and set aside. In a saucepan over medium high heat, combine water, sugar, cornstarch, salt, and lemon juice. Cook the mixture while stirring with a whisk constantly about 2-3 minutes. To temper the egg yolks, add about half a cup of the hot mixture into the yolks and whisk vigorously. This will make the eggs come to the same temperature without scrambling. Add the egg yolk mixture into the lemon mixture and cook for 3-4 more minutes. It will thicken. Remove from heat, and whisk in the lemon zest and butter cubes, the butter will make the filling silky and smooth.
Preheat the oven to 350°F/180°C.
Briefly whip the meringue again for 20-30 seconds.
Spread the meringue over the hot lemon curd filling making sure that you touch the edges of the pie crust to seal the lemon curd.
Bake the pie in the oven for 10 minutes or until the meringue topping is toasted. Allow to cool at room temp for 1.5-2 hours before slicing.