In bottom of crock pot, combine 1 cup barbecue sauce, onion and bouillon cubes. Place beef brisket on top. Cover; cook on low setting 6 to 7 hours or until brisket shreds easily. Remove brisket from crock pot; reserve 1 cup cooking juices. Remove fat layer and shred meat. Return meat to crock pot along with reserved meat juices and remaining barbecue sauce. Mix well; adding more barbecue sauce if needed. Cook meat mixture on high setting until hot. Serve on sandwich buns.