Optional: spice power. A variation on the traditional seven-spice power is to put out dainty dishes of the mixture's key ingredients: sesame seeds, chili powers, and tangerine zest. Add a pinch of each to the broth and stir.
As with much Japanese cuisine, the aesthetic presentation of soba is essential. This version uses ingredients that evoke late winter: bare earth (shiitake mushrooms), snow (tofu), evergreens (seaweed), and the hope of an early sign of spring (green onion).
Make the broth. Place 1/4 cup soy sauce, 1 tablespoon each brown sugar, chopped ginger and chopped garlic, plus 1 cup water, in a pot over high heat. Next, divide 20 shiitake mushrooms into stems and caps. When soy mixture is at a boil, add caps, reduce heat and simmer 10 minutes. Remove caps; reserve. Add stems plus 5 cups water, bring to boil, reduce heat and simmer covered, 30 minutes. Discard stems.
Cook the soba. Bring a pot of salted water to a boil (if the soba noodles have salt as an ingredient, omit it from the water), add a 10- to 11-ounce package of soba, reduce to a simmer, and cook until the noodles are firm but with no crunch (similar to al dente pasta), about 3 to 6 minutes. Drain and rinse well with cold water, then divide soba into four bowls. Using a twirling motion with tongs to form soba into nest shapes.
Spoon in tofu and mushrooms. The Japanese prefer silken tofu, which has a custard-like texture. Drain a 1-pound container of silken tofu, place in bowl, heat in microwave on high for 1 minute, then scoop out with a tablespoon, dividing equally among bowls. "Tile" reserved mushroom caps off-center on top of tofu.
Sprinkle seaweed and scallions. Nori, the dark-green pressed seaweed in sushi rolls, gives this dish a hint of the sea. Cut a half sheet of it into matchstick-size strips with scissors. Sprinkle over tofu. Slice two green onions on an extreme bias; divide evenly among bowls.
Ladle in the broth. Make sure broth is extremely hot. To avoid disturbing the other ingredients, ladle soup slowly down the side of each bowl, as you would pour a glass of champagne.
Add a pinch of sesame seeds, chili powder, and tangerine zest to the broth and stir.