Heat the oven to 400 degrees.
Whisk together all the dry, powdery stuff (except the flour
for dusting, of course) in a large mixing bowl
Using your fingertips, rub the butter and shortening into the dry goods until the mixture resembles coarse crumbs.
Make a well in the middle of this mixture and pour in the buttermilk. Stir with a large spoon until the dough just comes together. Then knead in the bowl
until all the flour has been taken up.
Turn the dough out onto a lightly floured surface, then start folding the dough over on itself, gently kneading for 30 seconds or until the dough is soft and smooth.
Press the dough into a 3/4-inch-thick round. Using a 3-inch round cutter, cut out biscuits, being sure to push the cutter all the way through the dough to the work surface before twisting to punch out the biscuit. Make your cuts as close together as possible to limit waste.
Place the biscuits on an aluminum sheet pan so that they just barely touch. Reroll scraps and punch out as many biscuits as possible.
Use your thumb to create a shallow dimple in the top center of each biscuit, and bake until the biscuits are tall and light gold on top, 15 to 20 minutes. Turn the biscuits out into a kitchen towel-lined basket and let cool for several minutes before buttering and devouring.