Preheat the oven to 325 and line a pair of 8-inch cake pans with parchment so the shortbread can be easily lifted out.  Sally recommends using a square, cutting a 1-inch slit in the center of each side, and then pressing and folding each into the pans for fewer creases.

Using your electric mixer, beat together the butter and sugar for a couple of minutes until creamy, scrape down the sides if needed, then beat in the vanilla and salt.  Add in the flour and cornstarch and mix on low until the dry ingredients are mostly worked in, then turn the mixer up to medium for a minute or 2 until the dough is well mixed and started to clump..  If you’re adding any extras, mix them in now.

Divide and weigh the dough into 2 portions, then press each half evenly into your prepared pans.  Bake for 28-30 minutes until the tops and edges are lightly browned.  Allow the pans to rest for about 15 minutes before using the parchment to lift each out onto a cutting board.  Cut the shortbread into wedges, while still warm – 8, 12 or 16 – and then enjoy straight away or leave to cool completely on a rack before packaging and/or freezing them.