Though they aren't in the classic Girl Scout logo shape, these shortbread cookies are as close to their classic Trefoils as you can get.
Preheat oven to 300 degrees F. In large bowl, with mixer at low speed, beat butter and 1/3 cup sugar until blended. Increase speed to high and beat until light and creamy, occasionally scraping bowl with rubber spatula.
With hand, mix both kinds of flour into butter mixture just until crumbs form. (Do not overwork dough or shortbread will be tough.) Pat shortbread crumbs into ungreased 9-inch round tart pan with removable bottom or cake pan. With fork, prick dough all over in a decorative pattern. Sprinkle with remaining tablespoon of sugar.
Bake shortbread 1 hour 10 minutes or until lightly browned around the edge. Let cool in pan on wire rack 10 minutes.
Remove shortbread from pan to cutting board. While still warm, cut shortbread into 16 wedges. Cool wedges completely on wire rack. Store cookies in tightly covered container up to 1 week.