Preheat oven to 450 degrees F.

In a large mixing bowl, whisk together the flour, baking powder, baking soda, and salt. Using a pastry blender or fingertips, rub butter and shortening into dry ingredients until mixture looks like crumbs. (Do fast so fats dont melt.) Chill in fridge for 10 minutes. Or put in fridge the night before. Or, in a zip-top bag and frozen for up to 1 month.

Make a well in the center and pour in the chilled buttermilk. Stir just until the dough comes together. The dough will be sticky.

Turn dough onto floured surface, dust top with flour and gently fold dough over on itself 5 or 6 times. Press into a 1-inch thick round. Cut out biscuits with a 2-inch cutter, being sure to push straight down through the dough. Place biscuits on baking sheet so that they almost touch. Reform scrap dough, working it as little as possible and continue cutting. (Biscuits from the second pass will not be quite as light as the first...cest la vie.) Or, cut in squares to avoid scraps.

Brush tops with buttermilk. Bake 15 minutes or until biscuits are tall and light gold on top.

For bigger batches: 1 part Fat (for every 1 part fat) 3 parts Buttermilk 6 parts Flour 1 T baking powder 1/2 t baking soda