Cook noodles al dente according to package instructions. Drain, pat dry, and transfer to a large bowl.
In a small bowl combine peanut butter, water, soy sauce, sesame oil, and rice vinegar. Whisk until smooth. Add mixture to noodle bowl and toss to coat. Add carrots and green onions and mix to combine. Serve warm, cold, or at room temperature topped with sesame seeds if desired.
Notes
Add chicken, pork, or beef to turn this into a hearty main dish.