When ready to serve, toss noodles, chicken, sauce and remaining ingredients except peanuts. If you prefer, you can substitute standard, sweet basil, but the taste won't be the same. Lemon grass is sold in Asian markets and many supermarkets. Use the tender base of the bulbous stalk, usually about 4-6 inches. Peel off any tough, fibrous layers.
Fish sauce is sold in Asian markets and in the Asian specialty sections of some supermarkets.
Off heat, add rice noodles and let sit, stirring occasionally, until tender, 5-10 minutes. Drain and distribute among 4 bowls. Meanwhile, bring broth, garlic, ginger, star anise, fish sauce, soy sauce, sugar, lemon grass and shrimp shells to boil in medium saucepan over medium-high heat. Add shrimp and simmer until opaque and cooked through, about 2 minutes. Add chili and lime juice and season broth to taste with salt, if necessary.