Heat oil in a large skillet over medium-high heat. Add onion; sauté 5 minutes. Add ginger, garlic, and serrano; sauté 2 minutes. Stir in molasses. Remove from heat; carefully stir in rum. Cook mixture 2 minutes. Stir in brewed coffee, ground coffee, cinnamon, and cocoa. Bring to a boil; cook until reduced to 1 1/2 cups (about 20 minutes). Remove from heat; cool. Place mixture in a blender; process until smooth. Stir in butter and salt.
Trim fat from pork. Cut pork lengthwise into 8 1/2-inch-wide strips. Thread pork strips onto 8 (10-inch) skewers.
Place kebabs on grill rack coated with cooking spray; grill 4 minutes on each side or until desired degree of doneness, turning and basting frequently with coffee mixture.