Stir together the flour, powdered milk, baking powder, sugar, cream of tartar and salt in a medium bowl. Using a pastry cutter, cut the chilled butter bits and shortening into the dry mixture until blended thoroughly. Fold in the buttermilk until the dough comes together.
On a lightly floured surface, turn out the dough and use your hands to press it into a circle measuring 1 to 1 1/2 inches thick. Cut out circles with a biscuit cutter or a glass. Place the biscuits in cast-iron skillets, then brush the tops with the melted butter. Bake until golden brown, about 15 minutes.