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1
Combine the agar powder with the 300 ml of cold water in a small saucepan. Stir briefly to disperse the powder. Place over medium heat and bring to a full boil, stirring constantly. Maintain a rolling boil for 2–3 minutes, continuing to stir, until the liquid is entirely clear and no undissolved agar granules remain. The liquid should be transparent and slightly viscous; any remaining cloudiness indicates under-activated agar.
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2
Remove the pan from the heat. Add the sugar and the pinch of salt; stir until both are fully dissolved. Allow the mixture to cool, without refrigeration, until it reads approximately 70°C — the surface will have ceased active steaming and the liquid will feel distinctly hot but no longer scalding when a clean finger is passed briefly near it.
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3
This is where the dish lives or dies. Add the koshi-an to the agar base and stir with a flexible spatula until the paste is completely incorporated and the mixture is perfectly smooth and uniform in colour. Any residual lumpiness or streaking will set permanently into the finished jelly. The paste must be added at this temperature — no higher — to prevent protein denaturing in the bean paste, which would introduce a gritty texture irrecoverable in later steps.
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4
Rinse the mould with cold water; do not dry it. Continue to allow the mixture to cool, stirring gently, until it reaches approximately 60°C. Pour into the wetted mould in a single, steady motion. The surface should be smooth and level; any surface skin that forms indicates the mixture was poured too hot.
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5
Leave the mould at room temperature, undisturbed, for a minimum of 3–4 hours. The jelly will firm progressively; the edges will pull very slightly from the mould sides and the surface will no longer ripple when the mould is nudged. Transfer to the refrigerator for a further 1 hour minimum. Do not refrigerate while the jelly is still warm — condensation will form and the set will be irregular.
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6
To unmould, run a thin blade dampened with cold water around the interior edge of the mould and invert onto a clean board. To slice, rinse the blade in cold water, wipe to remove excess, and cut sections 1.5–2 cm thick in a single drawing motion. The wet blade glides through the tender gel without compressing or tearing; a clean edge and a slightly trembling, intact slice confirm correct set.