In a large, shallow bowl, whisk the soy sauce or tamari, sesame oil, peanut butter, vinegar, water, ginger, and garlic.
In a large pot, boil 3 quarts of water. Add the soba noodles and stir. Cook for 5 minutes, or until tender. Drain in a colander and run hot water over the noodles until the water runs clear. (If the noodles aren’t rinsed, they’ll become starchy and sticky.) Drain thoroughly again.
Add the noodles to the sauce and toss to coat evenly. Add the chicken and scallions and toss again. Garnish with the sesame seeds, peanuts, and cilantro.