Preheat oven to 325 degrees F.
Stir together sugar and cornstarch in medium-size saucepan until well blended. Gradually stir in lemon zest and juice until smooth. Mix in water and butter; cook over medium heat, stirring constantly, until mixture thickens and then boils for 1 minute.
Beat egg yolks slightly in small bowl. Whisk in about half of lemon juice mixture. Stir yolk mixture into lemon juice mixture in saucepan. Cook over medium-low heat, stirring constantly, 1 to 2 minutes until mixture thickens slightly. Pour into the pie shell.
Combine egg whites and cream of tartar in a medium-size bowl. Beat at medium speed until soft peaks form. Then gradually beat in sugar, 1 tablespoon at a time, and continue to beat until stiff peaks form.
Spread meringue over hot filling so it touches piecrust all around edge.
Bake in preheated 325 degree F oven until meringue is browned and temperature in center of meringue registers 140 degrees F on instant-read thermometer, 25 to 30 minutes. Remove to wire rack and cool to room temperature. Refrigerate until chilled, 4 hours or overnight. Serve cold. Makes 8 servings.