Preheat oven to 400°F. Place sweet potatoes on a tray and roast until soft, about 40 to 45 minutes. Let cool completely in skin, then peel. Either run potato flesh through a potato ricer or mash it until very smooth.
Preheat oven again to 400°F. Line a baking sheet with parchment paper and set aside. Whisk reserved sweet potato puree with buttermilk until smoothly combined.
In a large bowl, whisk flour, baking powder, sugar, spices, and salt together. Add the cold butter and use a pastry blender or your fingers to cut the butter into the flour mixture until it resembles a coarse meal.
Add the sweet potato mixture and stir until the dough comes together. Gently knead the dough into an even mass.
For marshmallow biscuits: Roll or pat dough out on a floured counter to a 1/2-inch thickness and divide evenly in half. Sprinkle marshmallows over half of the dough. Place the second half on top of the marshmallows and gently press together. For regular biscuits: Roll or pat dough out to a 1-inch thickness.
Dip a 2-inch biscuit cutter in flour and cut out biscuits by pressing straight down. Place biscuits on prepared sheet. Bake for 13 to 15 minutes until golden. Cool on rack.