oven to 325 degrees. For filling: Mix sugar, cornstarch, and salt in large heavy saucepan. Gradually stir in water and lemon juice until smooth. Add
egg yolks; stir until blended. Add
butter. Cook over medium heat, stirring constantly, until mixture thickens and comes to a boil. Reduce heat; simmer, stirring constantly, 1 minute. Remove from heat.
Stir in lemon peel; set aside. Immediatelymake meringue: Dissolve
cornstarch in cold water in small saucepan; bring to a boil, stirring constantly. Remove from heat;
egg whites and cream of tartar in mixer bowl with whisk attachment on high speed until foamy.Beating constantly, add
sugar, 1 Tbsp. at a time, beating after each addition until sugar is dissolved before adding the next. Continue beating until whites are glossy and stand in Soft Peaks
. Beating constantly, add
cornstarch paste, 1–2 Tbsp. at a time. Beat in vanilla. Pour hot filling into piecrust. Quickly spread
meringue evenly over filling, starting at edge and sealing to crust all around; swirl with back of spoon. Bakein upper third of 325 degree oven until lightly browned, 16–18 minutes. Cool on wire rack 30 minutes to 1 hour.