Bring enough water to just cover the sobanoodles to a boil. Once boiling, add the noodles and cook according to the package instructions – ours took 3 minutes. Once the noodles are done, drain off all but about 1 tablespoon of the cooking water.
Mix in the peanut butter, soy sauce, sesame oil, and Sriracha. Thin out the sauce if needed with a little water. Sprinkle with red pepper flakes and salt to taste.