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 all into a large resealable plastic bag; add steak. Seal bag and turn to coat; refrigerate for 8 hours or overnight, turning occasionally.
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 meat thermometer should read 145°; medium, 160°; well-done, 170°). Enjoy the steak.