In saucepan, stir cornstarch, salt and 1 cup sugar. Stir in 1 ½ c. water, lemon peel and juice. Cook over medium heat, stirring constantly, until lemon mixture is thickened & boils. Remove from heat. In small bowl with wire whisk, beat yolks. Stir in small amount of hot mixture. Slowly pour yolk mixture back into lemon mixture, stirring rapidly to prevent lumping. Return to heat, cook, stirring constantly until thick. DO NOT BOIL. Stir in butter. Pour into pie crust, cool 10 minutes. Preheat oven to 400°. In small bowl, with mixer at high speed, beat egg whites & cream of tartar. Gradually sprinkle in ½ c. sugar, 2 tbsp. at a time, until whites are stiff and glossy. With spatula, spread meringue over filling to edge of crust to keep from shrinking. Bake 10 minutes or until golden brown. Cool first, then refrigerate.