Boil the noodles: Bring a large pot of salted water to a boil. Cook 12 ounces spaghetti or lo mein noodles according to package directions until just al dente.
Shock them cold: Drain the noodles and rinse immediately under cold water, tossing until fully cooled. Drain well and set aside. This stops the cooking and keeps them from clumping.
Make the sauce: In a small bowl, whisk together 4 tablespoons soy sauce, 2 tablespoons rice vinegar, 2 tablespoons toasted sesame oil, 2 tablespoons creamy peanut butter, 2 garlic cloves, minced, 1 tablespoons honey or brown sugar, and 1 teaspoons hot sauce or sriracha (optional). Add 2 tablespoons warm water, to thin sauce a little at a time, whisking until smooth and pourable.
Toss it together: Add the cooled noodles to a large bowl. Pour the sauce over top and toss well to coat. Add 1 English cucumber, thinly sliced, 1 cups carrots, shredded, 1 cups shelled edamame, and 1 cups cooked chicken, shredded or chopped (optional) if using, and toss again.
Finish and serve: Top with 3 green onions, sliced and 1 tablespoons toasted sesame seeds. Serve right away, or cover and chill — it’s even better the next day. Toss with a splash of water before serving if it’s tightened up in the fridge.