In a saucepan, combine sugar and cornstarch. Gradually stir in water and cook over medium heat until thickened.

Remove from heat and stir in beaten egg yolks, lemon juice, lemon zest, and butter until smooth.

In a mixing bowl, beat egg whites and cream of tartar until soft peaks form.

Gradually add sugar and continue to beat until stiff peaks form.

Pour the lemon filling into the pie crust and spread the meringue on top.

Bake in a preheated oven at 350°F (175°C) for about 10 minutes or until meringue is golden.