Wanko Soba

Prepare Dashi: Combine kombu and 1000 grams of Water in a pot. Heat gently over medium-low heat, just below a simmer, for 20 minutes. Remove kombu just before Water boils. Bring Water to a boil, then turn off heat. Add katsuobushi and let steep for 5 minutes. Strain the dashi through a fine-mesh sieve, discarding katsuobushi.

Prepare Tsuyu: In a separate saucepan, combine Koikuchi soy sauce, Mirin, and Sake. Bring to a simmer over medium heat and cook for 2 minutes to evaporate alcohol. Add 500 grams of the prepared dashi to the saucepan. Bring to a gentle simmer, then remove from heat. Let the tsuyu cool to room temperature or slightly warm.

Prepare Scallion: Finely chop the Scallion and set aside.