In KitchenAid or with hand mixer beat butter until creamed.
Add brown sugar and beat until light and fluffy.
Stir in vanilla and egg yolk, pausing to scrape down the sides and mixing until ingredients are well-combined.
Add flour, gradually (about 1/2 cup at a time), pausing to scrape down sides of bowl. Halfway through the addition of the flour, sprinkle in the salt with the beater still running.
Chill dough for 30 minutes.
Preheat oven to 350F and line cookie sheets with parchment paper (if using. An ungreased, non-stick cookie sheet will also work).
Roll cookie dough into approximately 1" balls and use thumb to create indent/crater in the center. Space cookies at least 1" apart.
Bake cookies for 10-12 minutes and allow to cool completely on pan.