Preheat the oven to 300 degrees F. Place the butter and icing sugar in a mixing bowl and beat until light. Beat in the flour and cornstarch until well incorporated. Mix in the zest, vanilla, cranberries and chocolate chips. Flatten the dough into a 1/2-inch thick disc; wrap and refrigerate for 20 minutes. Roll out the dough on a lightly floured surface to a 1/4-inch thickness. Dip small cookie cutters in flour (this prevents the dough from sticking) and use them to cut out the cookies. Place on non-stick or parchment-lined baking sheets. Bake 15-18 minutes, or until very pale golden on the edges. Cool on a baking rack. Store in tight-sealing tins until ready to serve.