Place butter, brown sugar, vanilla, and salt in a large bowl. Beat together on medium speed until the mixture is very smooth, about 1 minute. Do not beat until light and fluffy. (See Tip 3)
Add the flour and mix on low speed only until the flour disappears into the dough, about 1 minute. Don't work the dough much once the flour is incorporated. (See Tip 3)
Fold in the toasted pecans with a wooden spoon or sturdy rubber spatula.
Transfer the dough to a gallon-size Ziploc bag. Put the bag on a flat surface. Leaving the top open, roll the dough into a ¼-inch thick rectangle that completely fills the bag. As you roll, turn the bag occasionally and lift the plastic from the dough so that it doesn't cause creases. Seal the bag, and place it in the refrigerator or freezer for at least 2 hours or for up to 2 days. (See Tip 4)
Preheat the oven to 325° F. Line a baking sheet with parchment paper or silicone mats.
Put the Ziploc bag on a cutting board and slit it open. Disc discard the bag and turn the firm dough onto a parchment-lined board or counter. Using a ruler as a guide and a pastry cutter, pizza cutter, or sharp knife, cut the dough into 1-½ inch squares. Transfer the squares to the baking sheets and carefully prick each one twice with a fork, gently pushing the tines through the cookies until they hit the baking sheet. Refrigerate for 20 minutes. (See Tip 5)
Bake at 325° F for 20 to 25 minutes or until golden brown. Remove from the oven and transfer the cookies to a rack to cool completely.
Yield: 3 dozen cookies. (See Tip 6)