Bake sweet potatoes until soft, and remove the skins to prepare for mashing. Mash until smooth, set aside, and allow to cool while creaming the butter and sugar in a large mixing bowl. Mix vigorously for a light and fluffy consistency before adding in one egg, mashed sweet potatoes, and a teaspoon of vanilla extract. Beat until fully blended.
In another bowl, combine flour, baking powder, baking soda, cinnamon, salt, and allspice. Slowly add in the butter and sugar mixture, and thoroughly mix before dropping in walnuts and raisins.
On an ungreased baking sheet, form tablespoon-sized balls from the dough and line up one inch apart. Bake at 375° for 10-12 minutes or until edges begin to brown, and remove from oven. Allow to cool on a wire rack, and then you're ready to serve!