Bring a large saucepan of water to a boil and cook the soba noodles for 4 minutes or according to the package directions. Meanwhile, make the dressing by mixing the peanut butter with the soy sauce, mirin, ginger and garlic. When noodles are done, drain and return to pan. Swirl in the sesame oil, then add the peanut mixture and toss the noodles so they're well coated.