Imo Yokan (Sweet Potato Jelly)

Peel and trim 250g sweet potato, cut it into 2cm rounds, and cover the pieces with cold water. Drain them after 10 minutes. Replace that water with fresh water and cook over medium heat. After 12 to 15 minutes, test the center with a skewer; it should pass through without resistance. Drain the potato promptly and press it through a sieve while it is still hot; cooling first makes the fibers harder to smooth out. Cover the sieved potato so its surface does not dry while the setting liquid is prepared. Combine 120ml water and 2g agar powder in a clean saucepan. Bring the liquid to a boil over medium heat, then stir for a full 2 minutes so no agar granules remain. Add 60g sugar and dissolve it completely before mixing in the sweet potato. Keep the pan over low heat for 2 minutes, working with a spatula until the puree and syrup form one even mass with no pale streaks or dry lumps. Dampen the mold with water, pack in the mixture, and level the top. Let visible steam dissipate before refrigerating it for at least 2 hours. Unmold the set block and cut it into four portions. A finished slice should keep a clean corner under the knife, retain the compact, soft grain of the sieved sweet potato, and avoid a rubbery gel texture.