Combine dry ingredients in mixing bowl. Cut butter into dry ingredients until mixture resembles cornmeal. Stir in the milk and sourdough batter and mix well, forming a ball. Knead gently on floured surface about 30 seconds. Roll dough about ½ inch thick.
Cut into circles with a cutter. Place apart on an ungreased baking sheet. Cover lightly and let rest for 30 min. Brush tops lightly with some melted butter and bake at 400° for 15 minutes or until biscuits have puffed and are golden brown.