Preheat oven to 325° F (165°C)
In a mixing bowl, cream the butter.
Add the sweetener and vanilla extract.
Sift the King Arthur Keto flour, if you haven't done so already.
Whisk the K.A flour and the cornstarch together.
Slowly add the flour and cornstarch to the creamed butter mixture.
If you find that the mixture is too dry, add a teaspoon of water and continue to mix. (Add up to one tablespoon of water)
Press the shortbread mixture into the bottom of an 8 inch by 8 inch baking dish, smoothing it out into an even layer.
Bake on the center rack of the oven for 20 minutes, or until just turning a golden colour.