To make lemon filling: In a medium saucepan, whisk together 1 cup sugar, flour, cornstarch and salt. Stir in water, lemon juice and lemon zest. Cook over medium-high heat, stirring frequently, until mixture comes to a boil. Stir in butter. Place egg yolks in a small bowl and gradually whisk in 1/2 cup of hot sugar mixture. Whisk egg yolk mixture back into remaining sugar mixture. Bring to a boil and continue to cook while stirring constantly until thick. Remove from heat. Pour filling into baked pastry shell.