In a separate bowl, combine the flour, salt, baking soda, and baking powder. Add to the butter mixture and mix until just combined. Roll into small balls and make an indent with your thumb in the center of the ball. Bake for 7-9 minutes or until dry looking and lightly cracked on the surface but still very soft. I left mine a little bit underdone so they were softer and fudgier, almost like a brownie.
Store at room temperature or in the freezer after the frosting has set.