Japanese sweet potatoes can be dry. They are sometimes even dry when they are in soup. I discovered a good way to cook Japanese sweet potatoes so that they are always moist and soft. This way is also really easy and convenient. I used the rice cooker. You can do this with regular sweet potatoes (さつまいも), the golden flesh kintoki imo (金時芋) and the orange flesh annou imo (安納芋).
How to cook Japanese sweet potatoes in a rice cooker
Wash and trim the ends (if necessary) of the sweet potatoes. Put 1-2 cm of water in the bottom of the rice cooker. Poke the sweet potatoes with a fork a few times just to make sure they don’t explode. Cook small or medium sized potatoes in a quick cycle (早炊き or 白米急速). Larger potatoes should be done with a full cycle. That is all.
Note: You can wrap the potatoes in foil, if you prefer. The ones I cooked while wrapped still had a soft skin so I am not sure you can get a crunchy skin in the rice cooker with water.