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1
Combine the kanten powder and water in a medium heavy-based saucepan. Set over medium heat, stirring constantly, until the kanten has dissolved entirely and the liquid is clear. No granules should be visible at the surface.
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2
Add the sugar and stir until fully dissolved. The liquid should remain clear and faintly syrupy.
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3
Add the anko in two or three additions, incorporating each fully before adding the next. Reduce the heat to low. This is where the dish lives or dies: stir continuously and reduce the mixture until it thickens visibly, pulls away from the sides of the pan, and the surface develops a glossy sheen (tsuya). Insufficient reduction at this stage produces a watery, unstable set; over-reduction seizes the paste. The mass is ready when a line drawn across the base of the pan with a spatula holds for two seconds before closing.
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4
Pour the hot mixture immediately into a lightly dampened 18 × 9 cm rectangular mould or loaf tin, smoothing the surface level with a dampened spatula. The surface should be flat and uniformly sheened.
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5
Allow to cool at room temperature until fully set and firm to a light press, then rest, uncovered, for a minimum of 24 hours. The block should hold a sharp, clean impression when pressed with a fingertip without springing back.
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6
Unmould onto a clean board. Using a sharp, thin-bladed knife — wiped clean between cuts — slice the block with a single downward stroke into portions approximately 1.5 cm thick. The cut edge should be perfectly smooth with no tearing or drag marks at the surface. Serve at room temperature to release aromatic compounds and achieve optimal texture.