You see a truck selling stone-baked sweet potatoes in Winter in Japan. There is a stone oven on a truck, and the sweet potato slowly baked on stones are the best, so sweet and smooth! The key is to bake slowly, not steaming or boiling so that you can maximize the level of sweetness. Also while you are baking, even keeping the skin, it lose some water thus the taste is more concentrated. I put a little of butter when I eat it. That’s the best way to eat sweet potatoes!
To get the similar effect of the stone baked sweet potatoes, here is what to do.
1. Heat the oven at 400F.
2. Put a sheet of aluminum foil on the metal tray.
3. Wash sweet potato to remove some bits of soil, and place them on the aluminum foil.
4. Baked for 1.5 hours. At 40-45 mins, flip the sweet potatoes. and bake for another 45 mins, or until the inside is soft enough. (You can check with a toothpick).