Combine flour and 1/3 cup sugar in bowl; cut in 1/4 cup butter until mixture resembles coarse crumbs. Place into ungreased 15x10x1-inch baking pan. Broil 3 to 4 inches from heat, watching closely and stirring every minute, 3-4 minutes or until golden brown and crunchy. Cool completely. Set aside.
Heat oven to 275°F. Generously spray 9-inch pie pan with no-stick cooking spray; set aside.
Beat egg whites, vanilla and cream of tartar in bowl at low speed until foamy. Add 3/4 cup sugar gradually, beating at high speed until stiff peaks form and mixture is glossy. Spoon into prepared pan; spread meringue over bottom and up sides.
Bake 1 hour without opening oven door. Turn off oven; let meringue shell stand in oven 1 additional hour. Remove from oven; cool to room temperature.
Combine water, 1 cup sugar and cornstarch in 2-quart saucepan. Cook over medium heat, stirring constantly, 5-7 minutes or until mixture comes to a boil. Continue cooking 1 minute. Gradually stir 1/2 cup sugar mixture into beaten egg yolks with whisk. Gradually stir egg mixture into remaining hot mixture. Continue cooking stirring constantly, 2-3 minutes or until mixture thickens. Remove from heat. Stir in lemonade concentrate, 1 tablespoon butter and salt. Cover; refrigerate 1 hour.
Stir sour cream into cooled filling mixture. Spoon cooled filling mixture into meringue shell; sprinkle with topping. Cover; refrigerate until serving time.