Heat a large, heavy skillet over medium heat; add pumpkinseeds. Cook 3 minutes or until lightly browned, shaking pan frequently. Remove from heat; cool completely.
Place pumpkinseeds, salt, cumin, cinnamon, and black pepper in a spice or coffee grinder; process until finely ground. Place spice mixture in a small bowl; stir in 1/2 cup broth. Set aside.
Heat skillet over medium-high heat; add tomatillos, chile, garlic, and onion. Cook 5 minutes or until browned, turning frequently. Remove from heat; cool. Quarter tomatillos. Trim root from onion half; discard root.
Place tomatillos, chile, garlic, and onion in a blender or food processor; process until coarsely chopped. Add 1/4 cup broth, cilantro, and lettuce; process until smooth.
Heat 1/2 teaspoon oil in skillet over medium heat. Add pureed tomatillo mixture; reduce heat to low, and cook 5 minutes or until slightly thick, stirring occasionally. Stir in spice mixture and 1/4 cup broth; simmer 10 minutes, stirring frequently. Add 1/4 cup broth, stirring with a whisk. Remove from heat; keep warm.
Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to 1/2-inch thickness using a meat mallet or rolling pin. Place chicken on grill rack coated with cooking spray; grill 3 minutes on each side or until done. Top chicken with mole. Serve with rice.