Arrange an oven rack in the lowest position and preheat the oven to 325°F.

Make the meringue. Use a stand mixer fitted with the whisk attachment or a hand mixer. In the mixing bowl, beat the egg whites and cream of tartar until frothy. Add the vanilla and then the sugar, 1 tablespoon at a time; until the sugar is incorporated and mixture forms glossy stiff peaks, about 2 minutes. Set aside while you make the filling.

Make the filling. In a medium bowl, whisk the egg yolks and sugar until thick and pale yellow. Add the cornstarch and salt and whisk to combine.

In a medium saucepan over medium heat, bring the water, lemon zest, and lemon juice just to a boil. Ladle about ¼ cup of the liquid into the egg mixture, and immediately whisk to combine. Pour the egg mixture into the saucepan with the remaining lemon mixture, and whisk constantly over medium heat. Cook until the mixture thickens and big bubbles begin to surface. Remove from the heat and stir in the butter.

Immediately scrape the lemon filling into the parbaked crust and spread evenly.

Top the lemon filling with the reserved meringue, spreading it all the way to the edges and touching the pie crust. Use the back of a large spoon to make decorative peaks on top of the meringue.

Bake until the pie is golden brown on top, 20-25 minutes.

Cool the pie at room temperature for 1 hour, then refrigerate for at least 4 hours until cold before slicing and serving, the same day the pie is baked.