Combine sugar, cornstarch and salt, add water gradually and cook over boiling water until smooth and thickened, stirring constantly. Cover and cook 15 minutes.
Beat egg yolks and pour hot mixture over them gradually, stirring constantly. Cook 5 minutes longer.
Just before removing from heat, add lemon juice, rind and butter. Mix well and cool. Pour into shell.
Meringue
Whisk all ingredients, then add on top of filling.