Cook the noodles according to the package directions. Rinse with cold water, drain, and toss with the sesame oil; reserve.
Meanwhile, peel and devein the shrimp; set the shells aside. Cook the shells in a large pot (dry) over medium until pink, 2 to 3 minutes. Add the chiles and garlic; cook 30 seconds more. Add the broth and fish sauce. Bring to a boil, then reduce to a simmer for 20 minutes. Remove the shells, chiles, and garlic and discard.
Add the peeled shrimp to the broth; cook until pink, 2 to 3 minutes. Remove the pot from the heat and stir in the salt, pepper, and half of the scallions and cilantro.
Divide the noodles and soup among 6 bowls. Serve with bean sprouts, peanuts, lime wedges, and the remaining scallions and cilantro.