Step 1
Bring 3 Tbsp. mirin (sweet Japanese rice wine), 3 Tbsp. soy sauce, 1 Tbsp. instant dashi powder (such as Hondashi), and 3 cups water to a boil in a small saucepan. Reduce heat so mixture is at a simmer and cook until flavors come together, about 4 minutes. Let mentsuyu cool, then transfer to an airtight container. Cover and chill until ready to use.
Step 2
Meanwhile, cook 6 oz. dried soba noodles (about 2 bundles) in a medium pot of boiling water according to package instructions (about 4 minutes is typically just right). Drain in a colander, then rinse under cold running water, taking care to remove any residual starch. Let sit 5 minutes.
Step 3
Divide noodles between 2 bowls. Ladle chilled mentsuyu into 2 smaller bowls and add an ice cube or two for an extra refreshing broth. Serve noodles topped with finely grated daikon, thinly sliced scallions, shredded toasted nori, and/or wasabi if desired. Dip a small amount of noodles in mentsuyu at a time, adding more ice cubes as you go as needed.
