Dissolve yeast in warm water. Let stand 5 minutes. Stir in the buttermilk: set aside. In a large bowl combine dry ingredients. Cut in shortening with a pastry blender until crumbly. Stir in yeast mixture; mix well. Turn onto lightly floured surface; knead lightly 3-4 times. Roll out to 1/2 inch thickness. Cut with 2 1/2 inch biscuit cutter. place on lightly greased baking sheets. Cover and let rise in a warm place for 1 1/2 hours. Bake at 450 degrees for 8 to 10 minutes. Brush tops with butter. Remove from pans to wire racks. Enjoy!