Look for soba noodles made from 100% buckwheat for the best flavor. If you cannot find Shaoxing wine, use sake or soju as a substitution — or omit this ingredient if you are avoiding alcohol.

Make the mentsuyu Stir together 4 cups water, bonito flakes, soy sauce, mirin, brown sugar, and shiitake in a medium saucepan over medium-low, and slowly bring to a simmer. Simmer until flavors ...