Combine first 3 ingredients. Cut in butter with a pastry blender until crumbly.
Stir together sweet potato and buttermilk; add to dry ingredients, stirring just until moistened.
Turn dough out onto a lightly floured surface, and knead 3 to 4 times. Pat or roll to 1/2-inch thickness. Cut dough with a 3-inch round cutter, and place biscuits on a lightly greased baking sheet.
Bake at 425° for 10 to 15 minutes or until golden.
Prep: 15 min., Bake: 15 min.