Prepare crust, bake and set aside to cool.
In a medium saucepan, over medium heat; combine sugar,cornstarch & flour, add 2 cups boiling water, cook and stir with a wire wisk over medium heat until thickened and bubbly, reduce heat & cook 2 minutes more.
Gradually add 1/2 cup hot mixture to egg yolks, then pour egg yolk/pudding mixture back into remaining filling in saucepan and cook another 2 minutes.
Remove from heat and add butter,lemon zest, vanilla, lemon extract and lemon juice, stir to combine.
Pour lemon mixture into cooked crust and set aside, allow to slightly cool.
In the mean time, prepare meringue by beating 4 egg whites, cream of tartar and vanilla until soft peaks form.
Then gradually add 1/2 cup sugar ( 1 tablespoon at a time), beat at high speed until stiff peaks form.
With a soft scrapper add large fluffy dollops of meringue on top of lemon filling, making sure to attach meringue to crust edges. This insures the meringue will not pull away from the crust.
Bake for about 15 minutes at 350 degrees or until meringue is golden brown, then allow to cool for about 2 hours.
Dip a serrated knife in cold water to slice through each slice.