Bring all ingredients to boil in large pot. Reduce heat; simmer uncovered until chicken is cooked through, about 30 minutes. Transfer chicken to bowl; cover to keep warm. Reserve chicken broth in pot; spoon off fat from surface.
Heat heavy large skillet over medium-low heat. Add pepitas; stir frequently until seeds puff and begin to pop, about 15 minutes (do not brown). Transfer to dish; cool. Set aside 2 tablespoons pepitas for garnish. Working in batches, finely grind remaining pepitas with peppercorns in spice mill or coffee grinder.
Puree tomatillos, next 5 ingredients, and 1/2 cup reserved chicken broth in blender until almost smooth. Heat 2 tablespoons oil in heavy medium skillet over medium-high heat. Add tomatillo mixture and simmer until sauce is thick and reduced to 1 cup, stirring frequently, about 5 minutes.
Heat remaining 4 tablespoons oil in heavy large pot over medium heat. Add ground pepita mixture. Stir constantly until mixture resembles very coarse paste and begins to color in spots, about 9 minutes. Add tomatillo mixture; stir 1 minute. Add 2 cups reserved chicken broth and bring to boil. Reduce heat to medium-low and simmer until sauce is thick, stirring constantly, about 3 minutes longer. Season sauce to taste with salt. Spoon some sauce onto platter. Top with chicken. Spoon remaining sauce over. Garnish with reserved 2 tablespoons pepitas.
