Preheat oven to 325 degrees F. Place the baked pie crust on a rimmed baking sheet to catch any drips once filled.

For the lemon custard: In a medium sauce pot, whisk together the granulated sugar, cornstarch and salt until no clumps remain. Add in the egg yolks, water, and lemon juice and whisk until combined. Cook the mixture over medium heat, stirring constantly, until mixture thickens and begins to boil, about 10 minutes. Boil for 1 minute, stirring constantly, then remove from the heat and add in the butter and lemon zest, stirring until combined. Pour the custard into the prepared pie crust.

For the meringue: In a very clean and dry bowl of a stand mixer, combine the egg whites and cream of tartar together on HIGH speed with the whisk attachment, just until foamy. Gradually add in the sugar, about one Tablespoon at a time, beating until the mixture forms stiff peaks, about 2-4 minutes. Add in the vanilla and beat well.

Spread the meringue quickly over the lemon custard, sealing the custard under a layer of meringue. Pop the pie into the oven and bake for 20-25 minutes, watching closely so the meringue doesn't burn. It should be light golden brown and toasted. Cool completely at room temperature, then refrigerate for at least 3-4 hours to set before serving.