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1
Scoop the ice cream into four equal balls of approximately 100 ml each. Place on a tray lined with baking paper and return to the freezer. The filling must be completely firm — at least 2 hours — before wrapping begins. Press the surface of each scoop: it must feel stone-hard and yield nothing to the finger.
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2
Whisk the shiratamako, sugar, and cold water together in a microwave-safe bowl until completely smooth and no dry lumps remain. Cover with film and microwave on medium power in 90-second intervals, stirring between each, until the dough turns from opaque white to a translucent, glistening grey-white and pulls cleanly from the bowl wall. The smell will be faintly sweet and starchy. Total cooking time is typically 4–6 minutes depending on the oven.
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3
Dust a clean work surface generously with katakuriko. Turn the hot dough out immediately. Dust the top surface and, working quickly, spread and fold the dough several times until it is uniformly smooth, elastic, and no longer sticky to a dusted hand. The dough must be worked while still warm — it stiffens as it cools and will crack if allowed to cool before shaping.
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4
Divide the dough into four equal portions. Working one portion at a time and keeping the others covered, press each piece into a round approximately 12 cm in diameter and 4–5 mm thick. The disc must be neither so thin that it tears when stretched over the ice cream nor so thick that it overwhelms the filling in the mouth. This is where the dish lives or dies: too thin, the shell breaks and the ice cream is lost; too thick, the mochi dominates and the thermal contrast — the defining pleasure of the form — is buried.
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5
Remove the ice cream balls from the freezer one at a time. Centre a ball on a mochi disc. Gather the edges up and over, pinching firmly to seal at the base. Dust the finished piece with katakuriko and place seam-side down on a clean tray. Return immediately to the freezer if not serving at once.
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6
Aisu manju must be served within 2–3 minutes of removal from the freezer: the mochi shell hardens quickly and loses its chewy-elastic quality as it freezes solid. If serving from frozen, allow to temper at room temperature for no more than 2–3 minutes. The mochi should feel pliable and yield to a light press while the interior remains cold and firm — the two textures must be simultaneously present at the moment of eating.