Grind coriander, peppercorns, cumin, fennel, cloves, turmeric, and chiles coarsely in a spice grinder. Peel tough outer layers from lemongrass, then mash core with a meat mallet or small, heavy frying pan.
Heat oil in a large pot over medium heat. Add chicken, shrimp paste, shallots, and reserved spices and cook, stirring constantly, until fragrant, 2 minutes.
Pour in coconut milk, broth, sugar, and salt; add cinnamon and lemongrass. Bring to a boil, then simmer, covered, 20 minutes.
Boil bean sprouts in a large pot of boiling water until softened, 2 minutes. Transfer sprouts to a bowl. Add noodles to pot and cook until firm, 4 minutes. Drain; rinse well.
Divide sprouts and noodles among 6 bowls. Ladle in soup (remove cinnamon and lemongrass) and top with mint and cilantro. Serve with limes and sambal.
*Find in the Asian aisle of a well-stocked grocery store or at an Asian market.
Note: Nutritional analysis is per 1 1/2-cup serving.