Soba is the Japanese name for buckwheat and it also means the noodles made from buckwheat flour. I've seen all kinds of creative soba noodle recipes in the U.S., but in Japan soba noodles are served either simply chilled with a dipping sauce or in hot broth as a noodle soup, similar to how we prepare Japanese udon noodle soup.
Cold soba noodles are typically served in zaru, which means bamboo basket in Japanese. We serve noodles with some toppings (green onions and wasabi, sometimes grated daikon) and a dipping sauce called mentsuyu.
In a medium-sized saucepan, combine the water and dashi packet. Cover with a lid and cook over medium-high heat. Once it reaches a boil, reduce the heat to medium-low and simmer for 5 minutes. Discard the packet. Pour out 1 ½ cups of the stock and reserve for another use.
Add the mirin, soy sauce, and salt to the remaining stock and bring to a boil. Add more dashi stock if the sauce is too thick. Remove from the heat and let cool. (The dipping sauce is supposed to be a little salty because you will be "dipping" the soba noodles instead of soaking them to eat. You can always dilute it later if needed.)
Bring a large pot of water to a boil over high heat. Do not salt the water. Add the dried soba in a circular motion, separating the noodles from each other. Boil according to the package directions. Stir occasionally so they don't stick to each other. Do not overcook.
Drain the noodles in a colander and wash the noodles in a cold running water — this is very important and key to great tasting soba noodles.
Put the scallions and wasabi on a small plate. Serve the soba noodles on a tray or dish. Top with finely shredded nori sheets right before serving.