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1
Remove the kirimochi blocks from the refrigerator immediately before frying. Pat each block thoroughly dry with kitchen paper — every trace of surface moisture must be removed. Any residual moisture will cause violent oil spatter. The blocks should feel dry and firm to the touch.
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2
Heat the oil in a deep, heavy vessel to 180 °C. Confirm the temperature with a thermometer. Lower the cold, dry kirimochi gently into the oil. Within 2–3 minutes the mochi will begin to puff dramatically, doubling or tripling in volume as the temperature contrast between the refrigerator-cold block and the hot oil drives rapid expansion. This is where the dish lives or dies: the mochi must enter the oil cold and dry, at exactly 180 °C — warm mochi puffs incompletely; wet mochi spatters. Fry for 3–4 minutes in total until the exterior is golden-brown, blistered, and crackled across its entire surface. A leathery, uncrackled surface indicates under-frying; this exterior will not absorb broth properly.
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3
While the mochi fries, bring the dashi to a simmer. Add the mirin and soy sauce. Taste: the broth should read slightly sweeter than a standard udon tsuyu, balancing the rice cake's neutral starchiness.
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4
Cook the udon noodles according to their form — fresh noodles require only 1–2 minutes in boiling water. Drain and portion into warmed bowls. Ladle the hot broth over the noodles. The noodles should be fully submerged and the broth steaming vigorously.
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5
Lift the age-mochi from the oil, drain briefly, and place one piece on top of the noodles in each bowl immediately before the bowl goes to the table. The eating window of optimal crisp-then-soft contrast is 2–3 minutes from this moment. Once the mochi has fully softened and the crackled exterior crust is lost, the defining textural contrast of the dish is gone — serve and eat without delay.