Place flour, powdered sugar, salt, xanthan gum, pine nuts and zest in food processor. Pulse a few times to mix. Cut cold/frozen butter into ½ inch slices and add to food processor; pulse until butter is the size of small peas. Press this mixture evenly into a foil lined 9x13x1 inch baking pan which has been coated with cooking oil spray. Bake 15 to 20 minutes; until crust is just starting to turn golden brown.