Preheat oven to 325°F.
Measure flour, ¼ cup granulated sugar, powdered sugar, and salt into the bowl of a large food processor. Pulse until blended. Add butter and pulse in long pulses until the mixture resembles coarse sand.
Pour the mixture onto a lightly floured board and, with floured hands, gently form it into a ball. Press dough into the bottom of an ungreased 9-inch baking pan with a removable bottom, or a 9-inch springform pan. Use the tines of a fork to make impressions around the border of the dough, and then evenly prick the surface. Sprinkle with the remaining 1 tablespoon of sugar.
Bake for 35 to 40 minutes or until pale golden brown with a firm center. Remove from the oven and cut into 8 to 12 wedges with a sharp knife. Allow to cool completely before removing sides of pan and lifting out cookies. Store in an airtight container.