Satsuma Imo or Sweet Potatoes are often used in Japanese cakes/Wagashi.
The great advantage is that it makes for completely vegan cakes with an almost endless source of variations.

Here is the basic recipe from you will be able to improvise!

INGREDIENTS:

-Satsuma/Sweet Potato: 400g (peeled)
-Sugar: 75 g
-Agar agar powder: 3 g
-Salt: a pinch
-Water: 20 ml

RECIPE:

-Cut the sweet potato into small pieces and wash under clear cold water to take off astringency.
Boil in a pan with 20 ml of water until soft.

-Just before the potatoes are completely cooked, add sugar agar agar and salt. Bring to boil and swith off fire. Bear in mind there will is very little water. Do not burn the poatoes!

-Transfer potatoes into a frying pan and fry until they get smooth..

-Return to boiling pan and heat to get all excess water out.

-Pass through a sieve, or process.

-Wet the inside of a refrigerator recipient before spreading cellophane paper inside. Pour the potato puree into the recipient and fold the cellophane paer on top, leaving no air between cellophane paper and potato. Chill inside refrigerator.

Before eating unwrap cellophane paper and cut into preferred shape.
This is where the fun begins!