Heat a large heavy skillet over moderate heat until hot and toast pumpkin seeds, stirring constantly, until they have expanded and begin to pop, 3 to 5 minutes. Transfer seeds to a plate to cool. In skillet heat sesame and cumin seeds, peppercorns, allspice, and cloves, stirring, until fragrant, about 1 minute, and transfer to the plate. When seeds and spices are cool, grind them in batches in a clean electric spice/coffee grinder.
If using fresh tomatillos, discard husks and rinse with warm water to remove stickiness. Stem serrano chilies. In a saucepan simmer fresh tomatillos and serranos in salted water to cover 10 minutes. (If using canned tomatillos, drain them and leave serranos uncooked.) Transfer tomatillos and serranos (use a slotted spoon if simmered) to a blender and puree with onion, garlic, 1/4 cup cilantro and salt until completely smooth.
Heat the oil in a 5-quart heavy pot over medium-high heat. Pour in the tomatillo puree and cook, stirring frequently, about 10 minutes. Add 2 1/2 cups of the broth and stir in the powdered pumpkin-seed mixture. Simmer sauce, stirring occasionally, until slightly thickened, about 15 minutes.
In blender, puree 1/2 cup broth and remaining 1/4 cup cilantro until completely smooth. Stir puree and chicken into sauce and heat until hot.
Transfer the chicken mixture to a bowl or plate and garnish with pumpkin seeds and cilantro.