Japanese soba noodles are lower in calories than regular pasta, with almost as much protein; their assertive flavor is matched here with rice vinegar, soy, and fresh cilantro.
Bring a large pot of salted water to a boil. Cook noodles until al dente, according to package instructions. Drain, rinse under cold water, and drain well again.
In a large bowl, combine noodles, cilantro, vinegar, soy sauce, and, if using, chili oil. Season with salt and pepper; toss to combine. Serve sprinkled with sesame seeds.