Sift the flour, baking powder and salt in a large bowl.
Cut in the chilled butter and shortening with a pastry cutter or pulse in a food processor until the mixture resembles coarse, crumbly meal. Stir in the Mashed Sweet Potatoes and knead until the dough holds together.
On a lightly floured surface, knead gently for 1 minute, adding a little flour as necessary to incorporate all the ingredients. Pat the dough into a 1/2 inch thick circle and let it rest, covered with a clean towel, for 10 to 15 minutes.
Cut out the biscuits with a 2-inch round cutter dipped in flour. Gather the scraps, pat out again, and cut into biscuits. Arrange them about 3/4-inch apart on an ungreased baking sheet and brush the tops with milk.
Bake until golden brown, 10 to 12 minutes. Cool biscuits on a rack before serving.