Bring a large pot of water to a boil. Cook noodles according to package directions. When it's time to drain them, reserve about 2 tablespoons of pasta water to thin out the sauce (you may not need all of it).
While the pasta is cooking, make the sauce. Whisk together sesame seeds, peanut butter, garlic, ginger, soy sauce, vinegar and brown sugar until smooth. Drizzle in a little pasta water and whisk again. Add water until the sauce has thinned out to your desired consistency.
Drain pasta then add to a bowl. Drizzle with sesame oil and toss until coated. Add peanut sauce to the noodles and toss again. Top with green onion and red pepper flakes if desired. Add grilled chicken if you want more protein.