Coat a 9-inch round tart pan with a removable bottom with nonstick spray.
In a bowl of a standing mixer fitted with a paddle attachment or in a large mixing bowl with a rubber spatula (or wooden spoon), combine all the crust ingredients: the butter, all-purpose flour, white whole wheat flour, powdered sugar, salt, vanilla, water, and lemon zest (if using). Mix until a somewhat dry looking cookie-dough-like mixture forms; when pressed together with your fingers, it should hold together. Press evenly along the bottom and up the sides of the prepared tart pan. Trim off any excess dough that overhangs the pan. Prick the dough all over with the tines of a fork.
Freeze the crust for 15 minutes. Meanwhile, place a rack in the center of the oven and preheat to 350 degrees F. Bake for 18 to 20 minutes, until golden. Let cool completely, then use as desired.
Notes
TOSTORE: Shortbread pie crust can be stored at room temperature. Wrap tightly with plastic wrap to ensure it does not dry out.
TO FREEZE: You can freeze shortbread dough before baking it or after baking it. Tightly wrap and keep in freezer for up to 3 months. Before using, let thaw overnight in the refrigerator.