1. Preheat the oven to 325ºF and lightly grease a 9-inch round fluted pan.
2. Cream the butter and brown sugar together with the vanilla until light & fluffy. Sift the flour, cornstarch and salt over the bowl and stir in the flour until evenly incorporated. Stir in the chocolate chips. Spoon the dough into the prepared pan and press flat. Dock with a fork and then bake for about 50 minutes, until a golden brown and a rich golden brown at the edges.
3. Let the shortbread cool for 5 minutes, then slice it into 16 wedges before cooling completely.
4. The shortbread will keep in an airtight container for up to a week.