Sift flour, cornstarch and salt into a mixing bowl. Mix and set aside. In another bowl, cream butter with icing sugar. Add egg and orange zest and mix well to distribute the zest. Mix in ground almonds. Stir in flour mixture to form a dough.
Press dough together and roll into a long tube about 1 1/2-inch (4 cm) in diameter. Wrap in parchment paper, then plastic wrap. Refrigerate for at least 1 hour or overnight, or freeze up to 2 months.
Preheat oven to 300°F (150°C). Unwrap and slice cookies 1/4-inch (5 mm) thick, pressing one slivered almond into the centre of each cookie. Bake on a parchment paper-lined tray for 12 to 15 min., or until cookie edges are lightly browned. Cool on wire rack and store in an airtight container for up to 2 weeks.