In a large resealable plastic bag, combine vinegar, cocoa, brown sugar, olive oil, cumin, chili powder, salt, and pepper. Add flank steak; seal bag, and refrigerate for 1 to 2 hours or overnight.
Spray nonstick griddle with nonstick spray and preheat to medium-high.
Remove steak from marinade and discard marinade. Place steak on the griddle and cook for 6 minutes on each side for a medium-rare. When cooked to desired doneness, remove from the griddle, to a cutting board, and let rest for 10 minutes. Cut steak across the grain into thin strips. Serve with Pineapple Salsa and garnish with chopped cilantro, if desired.
In a medium bowl, combine pineapple, avocado, onion, cilantro, lime juice and salt. Cover, and refrigerate for 1 hour.