Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add sweet potato, cover, and steam until tender, 8 to 10 minutes. Strain into a bowl and mash until smooth.
Mix flour and salt together in a large bowl. Cut in butter with a pastry blender or a fork until mixture is crumbly. Stir in sugar, raisins, cinnamon, nutmeg, and allspice. Add 3 cups mashed sweet potatoes; mix until dough comes together.
Turn dough out onto a floured work surface; divide in half. Pat each half into a circle the size of a large dinner plate. Cut each circle into 8 wedges. Transfer wedges to two 10x15-inch baking sheets.
Bake in the preheated oven until lightly browned, about 20 minutes.