Place the cornstarch and water in a small saucepan over medium heat. Whisk constantly as the mixture thickens. Once the mixture begins to simmer, remove it from the heat.
Preheat the oven to 325℉.
In a small bowl, combine the sugar and cream of tartar. Set aside. Place the egg whites and vanilla in the bowl of your stand mixer and whisk until they are frothy. Whisk in the sugar mixture, one tablespoon at a time. Continue to beat until the whites reach soft peaks.
Add the cornstarch mixture one tablespoon at a time. Continue to beat until the meringue has reached stiff peaks. A minute or two before you hit this stage, place the lemon filling back over low heat, remove the plastic wrap and make sure it's warm before continuing to the next step.
Pour the hot lemon filling into the pre-baked pie shell. Immediately top with meringue. Dollop the meringue around the edge of the crust, then fill in the center. Be sure to attach the egg whites to the crust to prevent shrinking. Use a spoon or a spatula to create a shaggy look.
Bake for 20 minutes, or until the meringue is golden brown. Transfer to a cooling rack. Allow it to come to room temperature before serving.