Preheat the oven to 300ºF (150ºC).
Lightly butter a 9-inch tart ring or springform pan with a removable bottom.
(I used an open tart ring placed on a baking sheet covered with parchment paper.)
Whisk together the flour, corn starch, salt, and sugar in the bowl of a stand mixer. Add the butter and mix on low speed until the mixture starts coming together in clumps. Add the vanilla and continue to mix until the dough forms solid clumps.
Use the heel of you hand to press the dough evenly into the tart pan. You want the top to be a smooth as possible.
With a sharp knife, score the shortbread into twelve even wedges and prick each wedge three time with the tines of a fork.
Bake the shortbread for 45 to 55 minutes, or until the top is light golden brown. Remove from oven and immediately use a sharp knife to cut completely through the dough, where you previously marked it, into wedges. Let the shortbread cool completely in the pan. Once cool remove the outer ring of the tart pan, and separate the wedges.
Shortbread will keep for up to one week in an airtight container. It can also be frozen, if well-wrapped, for up to two months.