In a medium mixing bowl, sift together first 5 ingredients (through cream of tartar). Using a food processor or pastry cutter, cut butter into dry ingredients until butter is in pea-sized pieces. Using a fork, stir in buttermilk just until dough is uniformly moist (there will be lumps). Move dough to a large sheet of waxed paper; use a second sheet of waxed paper (not your hands) to fold dough in half a few times. Pat to an even 1" thickness, taking care not overwork the dough. Using a 2 ½" round cutter dipped in flour, cut straight down into dough to form biscuits (DO NOT twist cutter, and remember to re-dip the cutter in flour each time). If necessary, gather scraps of dough and quickly reknead, roll and cut remaining dough. In a preheated 450° oven, bake biscuits on a baking sheet lined with parchment paper until light brown on top (8-10 min.). Try them in our recipe for Smoky Chicken à la King or Sausage Gravy on Buttermilk Biscuits.