This is one of my wife and I's favorite meals for a hot day. It is very easy to make. All you need to do is make a dipping sauce, boil buckwheat noodles, and then provide toppings like grated radish and ginger.
To a medium pot, add the dashi stock, soy sauce, mirin, sake, and sugar. Bring to a boil, then reduce the heat to medium. Cook until the liquid reduces in volume by about half, about 15-20 minutes.
Taste the sauce and add extra seasonings as needed. Let it cool down to room temperature before serving. To do this quickly, I submerge the pot of sauce in a cold water bath.
In another pot, cook the buckwheat noodles according to the package instructions (usually this is about 3-4 minutes).
Drain the noodles and rinse a few times in cold water to bring the temperature down. Then transfer to a bowl of ice water for holding.
To serve, portion the noodles and dipping sauce into bowls, and provide the toppings on the side. Enjoy!