Heat the sesame oil in a large pot set over medium heat. Add the star anise, garlic, and ginger; heat until fragrant, about 30-60 seconds. Pour in the broth, shrimp shells, soy sauce, hoisin sauce, fish sauce, lime juice, and sliced jalapeño. Bring to a boil, then reduce the heat to low and simmer for 20 minutes with the pot covered 3/4 of the way.
While the broth is simmering, cook the noodles according to package instructions. Drain in a strainer, then rinse the noodles with cold water until they are no longer warm to prevent them from continuing to cook. (I also recommend tossing the noodles with a drizzle of oil, like sesame oil, to prevent them from sticking.)
Using a slotted spoon, carefully remove the shrimp shells and discard.
Add the bok choy and shrimp to the broth and cook for 3-4 minutes, until the shrimp are fully cooked and opaque.
Divide the noodles between bowls and ladle the soup over top, garnish and serve hot.
Notes
Shrimp shells can also be wrapped in a cheese cloth for easy removal.