Cranberry Orange Shortbread Cookies

Preheat the oven to 375 degrees F, and line baking sheets with parchment paper. Place the butter and sugar in the bowl of an electric stand mixer, and beat on medium-low speed until smooth. Add the egg and vanilla, and continue to beat until incorporated. Add 3 cups of the flour, the cornstarch, and salt, and stir on low speed until a stiff dough has formed. If the dough still feels sticky, add more flour (1/4 cup at a time) until it pulls away cleanly from the sides of the bowl. Stir in the dried cranberries and orange zest. Between 2 sheets of parchment paper, roll the dough to a thickness of 1/8-inch. Use a 2 1/2-inch fluted round cookie cutter to cut shapes. Place the unbaked cookies on the prepared baking sheets, and bake for 10 to 14 minutes, or until just beginning to turn golden around the edges.