Bring a large pot of water to a boil; add salt. Cook noodles until al dente, about 2 minutes. Drain; rinse with cold water. Set aside.
Bring coconut milk and chicken stock to a gentle simmer in a medium saucepan over medium-low heat. Meanwhile, process cilantro, basil, garlic, ginger, and 1 chile in a food processor until coarsely chopped, 5 seconds. Add cilantro mixture, fish sauce (if desired), lemongrass, and sugar to broth; simmer 6 minutes. Discard lemongrass.
Add noodles; cook until just heated through, about 1 minute. Remove from heat. Stir in lime juice. Finely chop remaining chile. Divide noodles among bowls; top with cilantro, basil, and chile.