In the bowl of a food processor (or in a medium mixing bowl if not using a food processor), lightly pulse the all-purpose flour, whole-wheat pastry flour, powdered sugar, ¼ teaspoon lemon zest, and salt. Add butter pieces and pulse just until mixture is crumbly and resembles coarse cornmeal. (If not using a food processor, cut butter pieces into the dry ingredients with a fork or pastry blender.) The mixture will feel dry to the touch. Press the mixture onto bottom of the prepared pan. Bake 20 to 22 minutes, until lightly browned. Set aside to cool for at least 15 minutes.
Preheat oven to 350 degrees F. Lightly grease an 8 x 8 inch pan with cooking spray.