Preheat oven to 350F. For the filling, whisk
together the cornstarch, salt, and 1 1/2 cups sugar in a medium saucepan
. Gradually stir in the water. Stir and cook over medium-high heat until bubbly and thickened. Cook 2 minutes more, stirring. Remove from heat.In a small bowl, beat egg yolks with a fork and and gradually stir about half of the hot filling into the yolks. Return mixture to the saucepan. Bring to a boil and cook 2 minutes more. Remove from heat. Stir in lemon zest
and butter, and then gently stir in lemon juice. Keep mixture warm. For the meringue, combine egg whites, cream of tartar and vanilla extract in a large bowl. Use an electric mixer to beat on medium speed until soft peaks form. Gradually add remaining sugar, 1 tablespoon at a time. Beat on high speed until sugar is dissolved and stiff peaks form. Pour filling pie shell. Spead meringue over the filling, sealing to edges. Bake
for 10 minutes or until peaks brown
. Cool for 1 hour on wire rack. Chill in the refrigerator for 3 to 6 hours before serving.