Preheat your oven to 325 degrees Fahrenheit. Line an eight-by-eight-inch baking dish with parchment paper and set aside.
In the bowl of a stand mixer (or in a large bowl), whisk together the flour, sugar, and salt.
1 3/4 cups all purpose flour, 2/3 cup confectioners sugar, 1/2 teaspoon table salt
Using the paddle attachment (or a hand mixer if using a bowl), beat in the butter one slice at a time on low speed (as soon as the piece of butter is well coated in flour, you can add the next piece - this step shouldn't take more than a minute or two). As soon as the butter is all added, add the vanilla. The dough will be very crumbly for a while and then start to form and pull away from the sides after about five to ten minutes (see the photos in the post).
14 tablespoons unsalted butter
Press the dough into the prepared pan in a smooth and even layer. Prick the top of the dough with a toothpick or skewer all over to allow steam to escape during baking.
Bake for 25 to 30 minutes until the edges just begin to turn a golden brown. Remove from the oven. Let cool for five to ten minutes. Then, using the parchment paper, remove the cookies from the pan and cut them into pieces while still warm. (See note 2.) I like to cut the cookies in half vertically and then cut each half into thirds. From there, I cut the cookies into thirds horizontally. This will leave you with 18 rectangles. Once they have cooled a bit more, transfer them to a rack to fully cool.