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1
Where a flavouring such as matcha or yomogi is used, combine it with the measured sugar first and work the two together until the mixture is uniform in colour and no streaks remain. Set aside.
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2
Sift the jōshinko and the kuzu or wheat starch into a mixing bowl. Add the sugar (plain, or pre-combined with the flavouring agent) and whisk to distribute. Pour in the cold water in a steady stream, whisking throughout, until the batter is smooth, fluid, and free of lumps.
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3
Prepare a wooden mould or bamboo container by lining it lightly with dampened muslin or parchment. Set up a steamer over briskly boiling water. Pour the batter into the prepared mould in one motion. This is where the dish lives or dies: steam over high, sustained heat until the centre is fully set — a skewer inserted at the centre comes out clean — and the surface feels springy rather than yielding when pressed lightly with a fingertip. Insufficient steaming produces a gummy, uncohesive interior that cannot be corrected after the fact.
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4
Remove the mould from the steamer. Wrap the uirō immediately in cellophane while still warm to prevent surface drying; a smooth, unmarked surface is part of the visual standard of the confection. Allow to cool completely at room temperature — a minimum of 4 to 6 hours — during which the flavour develops and the texture firms to its characteristic firm-chewy state.
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5
Once fully cooled and set, turn the uirō from the mould. Using a sharp, wetted knife, slice cleanly across the loaf to reveal the characteristic translucent-opaque cross-section. Hot or warm uirō tears at the cut; the blade should pass without resistance and the cut face should hold its shape without slumping.