01 -  In a medium saucepan, whisk together sugar, flour, cornstarch, and salt. 
 02 -  Stir in water, lemon juice, and lemon zest. Cook over medium-high heat, stirring frequently, until mixture comes to a boil. 
 03 -  Stir in butter. 
 04 -  Gradually whisk about 1/2 cup of the hot mixture into the beaten egg yolks to temper them, then slowly whisk the egg yolk mixture back into the saucepan. 
 05 -  Bring to a gentle boil and cook for 2 more minutes, stirring constantly, until thickened. 
 06 -  Remove from heat and pour the filling into the pre-baked pie crust. 
 07 -  Preheat oven to 350°F (175°C). 
 08 -  In a clean, dry bowl, beat egg whites and cream of tartar on medium speed until soft peaks form. 
 09 -  Gradually add sugar, 1 tablespoon at a time, beating until stiff, glossy peaks form. Beat in vanilla extract. 
 10 -  Spread the meringue over the hot lemon filling, sealing the edges at the crust to prevent shrinking. 
 11 -  Bake for 15-20 minutes, or until the meringue is golden brown. 
 12 -  Cool completely at room temperature, then chill in the refrigerator for at least 2 hours before serving.