In a large bowl, combine the flour, baking powder, baking soda and salt; cut in butter until mixture resembles coarse crumbs. Stir in enough buttermilk just to moisten dough., Turn onto a lightly floured surface; knead 3-4 times. Pat or roll to 3/4-in. thickness. Cut with a floured 2-1/2-in. biscuit cutter. Place in a large ungreased cast-iron skillet or other ovenproof skillet., Brush with milk. Bake at 450degrees until golden brown, 12-15 minutes.