Place a sweet potato on the cutting board between the handles of two wooden spoons. With a knife, cut the potato into 1/8-inch slices, being careful not to cut all the way through (the spoon handles will stop the knife just in time). Repeat with remaining potatoes.
Place the potatoes in a cast iron skillet. Add 4 tablespoons butter to the pan along with the cinnamon stick, cloves and allspice.
Sprinkle salt on each potato and top each with one tablespoon butter.
Place the skillet on the grill over indirect heat.
After 30 minutes, tilt the pan and spoon the melted butter over each potato.
Close the lid and continue grilling. Baste with the butter every 20-30 minutes until the potatoes are tender.
Smear marshmallow fluff over each potato. Sprinkle with brown sugar and pecans. Grill for another 5 minutes. Baste with butter for a final time. Remove from heat and serve.
You can bake these in a 400-degree oven, too.To make these on a campfire, wrap each potato in foil with butter, half a stick of cinnamon, cloves and allspice and place in hot coals until tender. Just before serving, open the foil pouches and top with marshmallow fluff, brown sugar and pecans and cook over the coals, until the fluff is melted.If you don't have whole spices, you can use ground.