Bring a large pot of water to a boil. Cook the noodles according to the package directions until just tender.
Drain the noodles and rinse them under cold water until completely cooled.
Shake off excess water well so the sauce does not become watery.
In a large mixing bowl, whisk together soy sauce, sesame oil, rice vinegar, peanut butter or tahini, honey or sugar, garlic, and chili oil or chili flakes if using.
Add cold water, 1 tablespoon at a time, until the sauce is smooth and pourable.
Add the cooled noodles to the bowl and toss until evenly coated with the sauce.
Add cucumber, carrot, green onions, sesame seeds, and cilantro if using.
Toss gently again until everything is well combined.
Taste and adjust with more soy sauce, vinegar, sesame oil, or chili oil if desired.
Serve cold or at room temperature.