1. Preheat oven to 425°. Stir together first 3 ingredients in a large bowl. Cut butter cubes and shortening into flour mixture with pastry blender or fork just until mixture resembles coarse meal. Cover and chill 10 minutes.
2. Whisk together buttermilk and sweet potato. Add to flour mixture, stirring just until dry ingredients are moistened.
3. Turn dough out onto a well floured surface, and knead lightly 3 or 4 times. Pat or roll dough to 3/4-inch thickness; cut with a 2-inch round cutter, reshaping scraps once. (Do not twist cutter.) Place rounds on a parchment paper-lined baking sheet.
4. Bake at 425° for 18 to 20 minutes or until biscuits are golden brown. Remove from oven, and brush tops of biscuits with melted butter. Serve immediately.