Preheat the oven to 350. Generously spray a 9-inch round cake pan or a tart pan (9 or 10 inches) with a removable bottom with baking spray. If using a cake pan, line the bottom with a circle of parchment paper. Set aside. If using a tart pan, line a sheet pan with foil and set it aside.

Combine soft butter, sugars, vanilla and salt in a medium-large bowl. Stir for about 30 seconds until nice and creamy.

Add the flour and cornstarch. Stir with a sturdy spatula or wooden spoon until all of the flour is incorporated and the dough starts to come together. Stir in the chocolate chips and mix until the chips are evenly incorporated.

Press the dough firmly into the prepared pan to create an even surface. If the dough is sticking to your fingers, dip your hand in flour from time to time. Sprinkle with the Demerara sugar.

If using a tart pan with a removable bottom, place it on the prepared sheet pan (sometimes a little butter may leak out) and place the sheet pan on the center oven rack. If using a round cake pan, place it right on the center rack. Bake for 25-35 minutes until nicely golden brown. Check frequently towards the end of baking time. You want it to be a pretty golden brown but not overbaked.

Turn off the oven and allow the shortbread to cool on a cooling rack for 20 minutes in the pan. Invert the shortbread onto a flat plate. Place another plate on top and invert again so the sugared side is up.

Slide the shortbread onto a cutting board and sprinkle with a bit more sugar, if desired. Cut it into 12 wedges

You can enjoy the shortbread right away but for really crispy shortbread, transfer the wedges to a sheet pan and place in the oven (still turned off). Allow the shortbread to cool completely in the oven then store in an airtight container.