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1
Make the kaeshi. Combine mirin and sugar in a small saucepan over low heat and stir until the sugar dissolves. Add the soy sauce and raise the heat gently until the mixture reaches 80 °C — do not allow it to boil, as boiling drives off volatile aromatics and flattens the flavour. Remove from the heat, cool to room temperature, decant into a sealed jar, and refrigerate for a minimum of three days before use. The kaeshi is ready when the soy, mirin, and sugar read as a single integrated note rather than three distinct ones.
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2
Assemble the tsuyu on the day of service. Combine cold dashi and aged kaeshi in a ratio of 4:1 by volume. Stir briefly and chill. Taste: the correctly concentrated dipping tsuyu should register as noticeably saltier and sweeter than you would serve as a soup — it is designed to dilute on the noodle surface. Adjust the kaeshi proportion accordingly.
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3
Prepare the ice bath. Fill a large bowl with cold water and a generous quantity of ice. It must be ready before the noodles enter the boiling water — delay here ruins the texture irreversibly. This is where the dish lives or dies: insufficient chilling leaves the soba clumped, sticky, and soft, destroying both the textural contrast and the dipping dynamic that define the preparation.
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4
Cook the soba in a large volume of rapidly boiling unsalted water — fresh nihachi soba requires 40–90 seconds; dried soba follows the packet time. The noodle is done when it yields a slight, definite resistance at the centre when bitten: katame, not soft. Err toward underdone; the ice bath will arrest cooking.
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5
Immediately transfer the cooked noodles to the ice bath. Agitate them with your hands to remove surface starch and to chill the strands through completely. The water will turn cloudy with expelled starch — this is correct. Drain thoroughly on the zaru. The noodles are ready when they feel firm and cool to the touch and no water pools beneath the rack.
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6
Arrange the drained noodles in a loose mound on each zaru. Cut the yaki-nori into thin ribbons immediately before service and lay them across the noodle surface — nori cut in advance and placed on cold wet noodles turns chewy and loses its ocean aroma. Serve the tsuyu in individual cups alongside small dishes of wasabi and grated daikon.
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7
Close the meal with soba-yu. Retain the hot soba cooking water and bring it to the table at the end of service. Each diner pours the hot soba-yu into their remaining tsuyu cup to dilute it into a warm, mineral-rich broth to be drunk. The liquid should be pale, faintly starchy, and warming — a gentle, saline close to the meal.