Combine flour, rice flour, sugar, ground ginger and butter pieces in food processor bowl with knife blade attachment; process 20 to 30 seconds. (To mix by hand - combine dry ingredients, then cut butter in with a pastry blender or rub butter in with fingers until no large pieces remain.) Particles should resemble coarse crumbs. Add minced crystallized ginger.
Firmly and evenly press mixture into an 8" or 9" round cake pan. Using the tines of a fork, make impressions around the edge of the dough, then prick surface evenly.
Bake at 325°F for 40 minutes or until pale brown in color.
While still hot, cut with a sharp knife into 16 wedges and if desired, sprinkle the surface lightly with a teaspoon of sugar. Leave in the pan to cool, then lift from pan and store in airtight container.