1. Preheat the oven to 230 degrees C.
2. Stir together the flour, powered milk, baking powder, sugar, salt, pepper, cream of tartar and sage in a medium bowl.
3. Using a pastry cutter, cut the butter and shortening into the dry mixture until blended thoroughly.
4. Stir in the buttermilk so the dough comes together.
5. On a lightly floured surface, turn out the dough and use hands to press into a circle 1 to 1 1/2 inches thick.
6. Cut out the circles with a biscuit cutter or glass. Place the biscuits in cast-iron skillets, then brush the tops with melted butter.
7. Bake in the oven until golden brown, 10 to 12 minutes.