In a medium saucepan over medium heat, whisk together sugar, cornstarch, and water. Cook until the mixture thickens and starts to bubble. Reduce heat and slowly whisk in the beaten egg yolks, stirring constantly. Add lemon juice, lemon zest, and butter, mixing until smooth. Remove from heat and pour the filling into the pre-baked pie crust.

In a clean mixing bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar and vanilla extract, continuing to beat until stiff peaks form.

Spread the meringue over the warm lemon filling, ensuring it touches the crust edges to prevent shrinking. Bake at 350°F (175°C) for 10–12 minutes, or until the meringue turns golden brown.

Allow the pie to cool at room temperature for 1 hour, then refrigerate for at least 3 hours before slicing. Enjoy chilled!