Begin by preparing the mentsuyu sauce. In a pot, combine soy sauce, mirin, sake, water, dried kelp, and dried pepper. Bring to a boil over medium heat, then reduce to a simmer. Simmer on low for 5 minutes.
Toss in the bonito flakes and once fully submerged, turn off the heat. Steep for at least 10 minutes before straining. Place in the fridge to cool completely.
While the mentsuyu sauce is cooling, prep the garnishes. Slice the cucumbers into match sticks. Transfer to a small mixing bowl and add salt. Toss to coat well then set aside for 10 minutes. After 10 minutes, squeeze the cucumber to remove excess liquid. Transfer to a clean bowl and mix in sesame seeds. Place in the fridge until ready to use. Cut the nori (seaweed) into thin strips. Chop the scallions. Set aside until ready to use.
Cook the soba noodles according to the package instructions. Once cooked, rinse well under cold water then strain. Transfer to a serving plate. Garnish with cucumbers and nori strips.
To serve, pour 1 to 1 1/2 cups of mentsuyu sauce into a serving bowl. Garnish with scallions and a few ice cubes to keep it chilled.
Dip the soba noodles into the mentsuyu sauce and enjoy!
