Heat oven to 425 degrees.
Sift the flour, baking powder, allspice, cloves, and salt in a large bowl. Use the tips of your fingers to blend in the brown sugar.
Cut in the chilled butter and shortening with a pastry cutter or pulse in a food processor until the mixture resembles coarse, crumbly meal. Next, stir in the mashed sweet potatoes and knead until the dough just holds together.
Turn the dough onto a lightly floured surface. Knead gently for 1 minute, adding a little flour as necessary to incorporate all the ingredients. Do not overwork.
Pat the dough in 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. Do not twist the cutter; press down then lift. 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, about 10 to 12 minutes. Cool the biscuits on a rack.