Japanese baked sweet potato. As the potatoes are usually roasted in a bed of gravel, the official name is "Ishi Yaki-imo." The potatoes used are long, red Satsuma-imo. By roasting them slowly and indirectly, amylase changes the starch in the potato into glucose, which is responsible for the delicate sweet taste. They are sold from street stalls and hawkers with small trucks and are a snack typical for winter.