Scottish Shortbread

Makes about 5 dozen 1 1/2-inch or 3 6- to 7-inch cookies. In large mixing bowl, beat butter, powdered sugar, vanilla, salt and orange zest until light. Fold in cornstarch, then flour, 1/2 cup at a time, blending well after each addition. Dough will be extremely soft. Spoon dough onto the center of a 16-inch length of plastic wrap. Fold long sides of plastic over dough. With palms, roll wrapped dough to form a log about 14 inches long and 1 1/2 inches in diameter. Twist ends of plastic wrap to seal. Refrigerate or freeze until firm, 1 to 4 hours. Or, to make large shortbread rounds: Before refrigerating, divide dough into 3 equal portions. Press each third evenly over bottom of a lightly buttered 8-inch pie pan. You can also form dough into 3 6- to 7-inch circles on lightly buttered baking sheets (2 on 1 sheet, 1 on the other). Refrigerate 1 hour. Score each circle into 8 wedges. Using the tines of a fork, prick the circles all over at 3/4-inch intervals. If desired, notch the edges of each circle with 1-inch-long slits, 1/4 inch apart. Preheat oven to 300 degrees F. Remove dough logs from refrigerator; unwrap. Cut chilled dough into 1/4-inch slices. Arrange, 1 inch apart, on ungreased baking sheets. If making large shortbread rounds, remove them from the refrigerator 5 minutes before baking. Bake 15 to 20 minutes (a few minutes longer may be necessary for the large rounds), or until pale golden. Cookies are very fragile. Use a metal spatula to carefully remove individual cookies from the baking sheets; cool on racks. Cool large racks in pans or on baking sheets. Store in an airtight container at room temperature 1 week; freeze for longer storage. Variations: Nutty Shortbread: Add 3/4 cup ground toasted almonds, pecans, walnuts or hazelnuts with flour. If desired, sprinkle top of unbaked shortbread with chopped nuts; press lightly onto surface. Chocolate Shortbread: Increase powdered sugar to 1 cup. Substitute 1/2 cup unsweetened cocoa powder, preferably Dutch process, for 1/3 cup flour. Spiced Chocolate Shortbread: Add 1/2 teaspoon each ground cinnamon and ground nutmeg to dry ingredients for Chocolate Shortbread. Spicy Shortbread: Add 1 teaspoon ground cinnamon and 1/2 teaspoon each ground nutmeg and ground allspice with flour. Chocolate-Dipped Shortbread: Melt 6 ounces semisweet chocolate with 2 teaspoons vegetable oil; cool until warm. Dip each cookie halfway into melted chocolate. Place dipped cookies on a waxed paper-lined baking sheet; refrigerate until chocolate is set. Shortbread Sandwiches: Spread about 1/2 teaspoon seedless raspberry (or your favorite) jam on the bottom of one shortbread cookie. Top with a second cookie. Chocolate-Dipped Shortbread Sandwiches: If desired, dip shortbread sandwiches halfway into melted semi-sweet chocolate