In a food processor mix the flour, mustard, salt and baking powder. Add the butter and process until the butter is pebble in sized pieces. Add the cheese and process. Add almost all of the milk and process until it comes together in a soft dough; do not overprocess. If dough is dry, add more milk.
Pat the dough out to 1/2inch thickness. Cut into square shaped biscuits or with a cutter or glass cut into 2 1/2-inch rounds. Set on a baking sheet and bake for 15 to 17 minutes.