Preheat the oven to 350 degrees.
To make the filling, whisk together 1 cup of sugar, flour, cornstarch, and salt in a medium saucepan.
Stir in the water, lemon juice, and lemon zest.
Cook over medium-high heat, stirring constantly until the mixture comes to a boil and thickens.
Remove from the heat. Pour the mixture into the baked pastry shell. Set aside.
For the meringue, in a glass or metal bowl, whip the egg whites until foamy.
Add the cream of tartar and continue mixing until soft peaks form.
Add 6 tablespoons of white sugar (1 tablespoon at a time), mixing between each tablespoon, until stiff peaks form.
Add vanilla extract and whip until just mixed in.
Spread the meringue over the pie filling, sealing the edges at the crust.
Bake in a preheated oven for 10 minutes or until the meringue is golden brown.
After baking, cool the pie away from drafts for 2 hours.
Cover and store in the refrigerator.