In a small skillet, toast sesame seeds in butter. Add onion and garlic; saute until tender. In a large bowl, combine the coffee, soy sauce, vinegar, Worcestershire sauce and sesame seed mixture. , Pour half into a shallow dish; add steak and turn to coat. Cover and refrigerate for 8 hours or overnight, turning occasionally. Cover and refrigerate remaining marinade., Drain and discard marinade from steak. Grill steak, covered, over medium-hot heat for 6-10 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Warm reserved marinade; serve with steak.