Preheat oven to 325ºF. Lightly butter a 8” square metal baking pan with a teaspoon of butter (or less). Cut a sheet of parchment the width of the pan and place it inside.
Cream the butter in the bowl of an electric mixer until light and fluffy, about 3 minutes, scraping down the bottom of the bowl and the beaters halfway through.
Add salt, powdered sugar, and vanilla extract and cream for an additional 2 minutes.
Add the all-purpose flour and almond flour and mix until fully combined, and then stop. Do not overmix.
Transfer the biscuit dough to the prepared pan. Using a small offset spatula spread the mixture evenly into the pan, smoothing over the top.
Bake in the preheated oven until the edges of the shortbread are lightly golden, about 45-55 minutes.
Let cool at least 2 hours before removing from the pan.
To remove, grab the parchment sling and evenly and carefully pull up to get the biscuits out.
Place the biscuits on a cutting board. Using a bread knife, slice the biscuits into rectangles, approximately 1” x 2.5”.
Cookies will keep, covered in an air-tight container at room temperature for 3-5 days.
