Combine first 4 ingredients in a bowl. Cut in 1/4 cup butter and the shortening with a pastry blender or forks until mixture is crumbly. Add mashed sweet potato and milk, stirring just until dry ingredients are moistened. Turn dough out onto a floured surface and knead just a few times.
Roll dough out to 1/2-inch thickness; cut with a 2-inch biscuit cutter. Place biscuits on an ungreased baking sheet and brush with the melted butter. Bake at 400° for 12 minutes, or until nicely browned.