Preheat oven to 400 degrees.
Combine the flour, sugar, cinnamon and nutmeg in a bowl.
Cut in the ¼ cup of butter and the shortening with a pastry blender or a fork until the mixture is crumbly.
Add mashed sweet potato and milk, stirring just until dry ingredients are moistened.
Sprinkle a bit of flour onto a flat surface. Turn the dough out onto the floured surface and knead just a few times. If the dough is sticking to the surface, add a bit more flour.
Use a rolling pin to roll the dough out to ½ inch thickness, then cut with a biscuit cutter or the mouth of a round glass. Add a bit of flour to the rolling pin or biscuit cutter when needed to keep the dough from sticking.
Place the biscuits on an ungreased cookie sheet and brush with melted butter.
Bake for 12 minutes. They don’t really brown on top, so check the bottom to make sure they are brown. Brush with butter and serve hot.