Place butter and icing sugar in a mixing bowl and beat until lightened. Gradually add flour and cornstarch and beat until smooth. Mix in a few drops of each food colouring to turn the dough a Halloween orange.
Roll the dough into 2-inch balls, and then place on a parchment-lined or non-stick cookie sheet. Gently push down into the centre of each ball to make it look squat like a pumpkin. Insert a half cashew in the centre for a stem. Refrigerate 30 minutes.
Preheat the oven to 325 degrees F. When the cookies have chilled 30 minutes, bake 15 to 18 minutes, or until the cookies are slightly firm to the touch and just cooked through. (Cookies should not take on any browning). Cool cookies to room temperature on baking rack. Store in a tight-sealing container until ready to serve.