Bring a large pot of salted water to a boil over high heat. Cook the spaghetti or other noodles to al dente according to their package instructions. Drain the noodles in a colander and rinse with cool water until they’re no longer hot.
While the noodles are cooking, make the sauce: In a large mixing bowl, whisk together the peanut butter, chili sauce, soy sauce, and boiling water (from the pasta pot). Continue to whisk until the ingredients combine into a smooth sauce, about 1 minute.
Taste the sauce and adjust the seasonings if you like, adding more soy sauce if you want it saltier, or more chili sauce for more sweetness.
Add the drained and cooled noodles to the bowl with the sauce. Use tongs to toss the noodles in the sauce until they’re evenly coated.
Transfer the peanut butter noodles to serving dishes. Top with sliced green onions, cilantro, and/or sesame seeds, if you like, and serve right away.
