Heat 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; cover. Beat 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.

Mix flour and salt in large bowl. Cut in shortening with pastry blender until pieces are the size of small peas. Mix egg yolk, 3 Tbsp. ice water, and the lemon juice in small bowl. Add to flour mixture, 1 Tbsp. at a time, stirring lightly and tossing with fork until moistened and dough just holds together. Add remaining ice water, if needed. Gather dough and shape into 1-inch thick disk. Refrigerate, wrapped in plastic wrap, 1 hour or overnight. Heat oven to 425 degrees. Roll out dough on lightly floured surface, rolling from center to edge, to form a 12-inch circle. Fold into quarters; ease and unfold into 9-inch pie plate. Press pastry gently against bottom and sides of pie plate, easing out any air. Trim edge leaving 1/2-inch overhang. Fold overhang under; flute edge. Refrigerate, covered, at least 30 minutes. Line piecrust with aluminum foil; fill with pie weights. Bake in 425 degree oven until pastry is set, 8–10 minutes. Remove foil and weights; pierce pastry all over with fork. Bake until pastry is golden brown, 5–7 minutes longer. Cool completely on wire rack.