preheat oven to 450°F

stir together the flour, powdered milk, sugar, baking powder, cream of tartar, and salt in medium bowl

using a pastry cutter, cut the butter and shortening into the dry mixture until blended

fold in the buttermilk until the dough comes together

on a lightly flowered surface, turn out the dough and use hands to press into a circle 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 melted butter

bake until golden brown, 10 - 12 minutes