Preheat the oven to 375°F.

If using a packaged frozen pie crust, follow the package directions to pre-bake. For a homemade crust, line with aluminum foil and fill with pie weights. Bake for 20 minutes, then remove foil and weights, poke the bottom with a fork, and bake for an additional 15 minutes.

Remove from oven and set aside.

Whisk the egg yolks in a medium bowl and set aside.

In a saucepan, combine cornstarch, sugar, salt, and water, whisk to combine and bring to a boil. Simmer until thickened.

Temper the egg yolks with the cornstarch mixture, then combine back with the pot. Cook for 3-4 minutes until thickened and add the lemon juice, zest, and butter.

Reduce the oven temperature to 325°F.

Whisk together cornstarch and water in a saucepan until thick, remove from heat.

Combine sugar and cream of tartar in a bowl, set aside.

Beat egg whites and vanilla until frothy, gradually add the sugar mixture, and beat until soft peaks form. Slowly add the cornstarch mixture and beat until stiff peaks form.

Re-heat the lemon filling until bubbling hot, pour into the pre-baked pie shell.

Quickly spread the meringue over the filling, ensuring it attaches to the crust. Create peaks with a spoon.

Bake at 325°F for 20 minutes until the meringue is golden brown. Cool completely on a wire rack.