Preheat the oven to 450 degrees. Combine the dry ingredients in a bowl or in the bowl of your food processor. Cut in the butter until it resembles coarse meal, or use a food processor to cut it in. Mix the sour cream and milk together, add to the mix and mix quickly to combine, using just enough strokes to make the dough come together. Turn the dough onto a floured board, knead one or two times gently, pat out about 3/4" thick. Cut the biscuits out with a biscuit cutter and place on the baking sheet. Bake for about 13-14 minutes, or until browned and crusty.