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1
Combine joshinko, sugar, and water in a bowl. Whisk until fully dissolved and no flour lumps remain. If using kuzu starch, substitute it for 30 g of the joshinko before mixing. If using a seasonal flavouring, incorporate it at this stage. The batter should be smooth, homogeneous, and pourable.
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2
Line a shallow rectangular mould or loaf tin with dampened parchment. Pour in the batter to an even depth. Set over a steamer of vigorously boiling water. This is where the dish lives or dies: steam on continuous high heat for 30 minutes without interruption — do not lift the lid, do not reduce the heat, do not pause. Any interruption to the steam produces a grainy, unset line through the cake at the point of interruption, a defect that cannot be corrected. The uiro is done when the surface is set, no longer liquid at the centre when tested with a skewer, and the cake has pulled very slightly from the sides of the mould.
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3
Remove the mould from the steamer. Allow to cool completely at room temperature — do not refrigerate at this stage, as cold arrests the final setting of the starch and dulls the characteristic yielding texture.
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4
Turn the cooled uiro out onto a board. Using a knife warmed in hot water and wiped dry, slice cleanly into rectangles. The blade must be re-warmed between cuts. The Nagoya-style block cuts cleanly and holds its form; a Kyoto-style kuzu version will be more translucent and yield slightly under the knife.