Prepare crust, bake, and let cool completely.

In a medium saucepan, combine sugar, cornstarch, and flour.

Add boiling water and stir with a wire whisk over medium heat until thickened and bubbly.

Reduce heat and cook for 2 minutes more.

Gradually add 1/2 cup of the hot mixture to the egg yolks.

Pour the egg yolk mixture back into the remaining filling in the saucepan.

Cook for another 2 minutes, stirring constantly.

Remove from heat and add butter, lemon zest, vanilla, lemon extract, and lemon juice.

Stir to combine thoroughly.

Pour the lemon mixture into the cooked crust.

Set aside and allow to cool slightly.

Prepare the meringue by beating egg whites, cream of tartar, and vanilla until soft peaks form.

Gradually add sugar, one tablespoon at a time, beating at high speed until stiff peaks form.

Using a spatula, add large dollops of meringue on top of the lemon filling, making sure to attach the meringue to the crust edges.

Bake at 350 degrees F (175 degrees C) for about 15 minutes, or until the meringue is golden brown.

Allow to cool for about 2 hours before slicing.