Roast whole sweet potato in the oven at 375 F. for 1 hour.Remove skins and slice into 1 inch thick rounds.In a large (12-15 inch) oven proof pot, over medium high heat, add butter, brown sugar and cinnamon and stir for 2 minutes. spread over the bottom of the pan and lay down the roasted sweet potatoes. Sprinkle the sweet potatoes with salt. Flip over the sweet potatoes. Then add marshmallows on top. Broil for 1-2 minutes in the oven (watch closely!) Remove from oven and serve. Enjoy!