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1
Bring a large pot of unsalted water to a full rolling boil. Cook the soba noodles according to the packet directions, typically 4–5 minutes, stirring occasionally to prevent the strands from clumping. The noodles are done when a single strand pressed between finger and thumb offers no raw, chalky core — tender throughout yet with faint resistance.
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2
Drain the noodles immediately into a colander. Rinse thoroughly under cold running water, rubbing the strands together gently to remove surface starch. For kake soba (hot service), briefly return the rinsed noodles to hot water to reheat before portioning into bowls. The noodles are ready when the rinse water runs clear and the strands are distinct rather than matted.
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3
Prepare the tsuyu. Combine the dashi, soy sauce, and mirin in a saucepan over medium heat. Bring just to a simmer, then remove from heat. For zaru soba (cold service), allow the tsuyu to cool to room temperature before pouring into individual dipping cups — on a December night, room-temperature tsuyu is preferred over chilled. For kake soba, ladle the hot tsuyu directly over the noodles in the bowl. This is where the dish lives or dies: tsuyu that is insufficiently balanced — too sharp with soy, too sweet with mirin, or thin for want of dashi — cannot be corrected at the bowl, and the spare toppings provide nothing to hide behind. Taste the tsuyu and adjust before it leaves the pot.
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4
Portion the noodles. For zaru soba, mound the cold noodles on bamboo draining trays or flat plates. For kake soba, place the noodles in deep bowls and pour the hot tsuyu over. Lay one slice of narutomaki (swirl pattern facing up), one slice of kamaboko, and one piece of nori on each portion. The bowl is ready when toppings are set without crowding — simplicity is the intent.
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5
Serve before midnight on December 31. The toshikoshi meaning is extinguished if the noodles are eaten after the year has turned. The dish should reach the table with enough time to eat without haste.