Preheat your oven to 350°F (175°C).In a saucepan, whisk together sugar, cornstarch, water, lemon juice, lemon zest, egg yolks, and butter over medium heat.
Stir constantly until the mixture thickens, then remove from heat and stir in vanilla extract.
Pour the lemon filling into the pre-baked pie crust. In a clean mixing bowl, beat egg whites, cream of tartar, and salt until soft peaks form.
Gradually add sugar while continuing to beat until stiff peaks form. Spread the meringue over the lemon filling, making sure to seal the edges.
Bake the pie in the preheated oven for about 10-12 minutes or until the meringue is golden brown.
Allow the pie to cool completely before serving. For best results, chill in the refrigerator for at least 2 hours before serving.