-
1
Prepare the steamer: bring water to a full boil over high heat and maintain it there. Line an 18 cm square or loaf mold with parchment and dust lightly with rice flour. The steamer must be at temperature before the batter enters — a cold or merely warm chamber will cause uneven gelatinization and dense pockets.
-
2
Combine joshinko and sugar in a bowl. Add the water in a thin stream, whisking continuously until a smooth, lump-free batter forms. If using a flavoring — matcha, sesame paste, sakura, or kuromitsu — whisk it into the batter now, before steaming, so it distributes uniformly throughout the block rather than as inclusions. The batter should be fluid and even in color with no streaks.
-
3
Pour the batter into the prepared mold. Set the mold in the steamer and cover with the lid — do not open it during cooking. Steam over sustained high heat for 30–40 minutes. This is where the dish lives or dies: under-steaming leaves raw starch at the center; a temperature drop from an opened lid causes collapse and dense pockets; the full period of uninterrupted heat is required for complete gelatinization throughout. The surface turns from opaque-white to a slightly translucent, set appearance with no wet shimmer at the center.
-
4
Remove the mold from the steamer. Allow the uiro to cool completely at room temperature, undisturbed, until the block is firm and stable to the touch — the texture continues to stabilize as it cools. Do not cut while hot: the interior is still mobile and the edges will tear rather than slice.
-
5
Turn the cooled block onto a board. Using a wet knife or a taut length of thread, slice into pieces 2–3 cm thick with a single clean draw per cut rather than a sawing motion. The sliced edge should be smooth and hold its form without crumbling. Wrap cut slices in plastic immediately to prevent surface hardening.