Gerard Butler gave an impromptu concert at Chelsea supper club The Darby last week. The “300” star was having a quiet dinner with friends, spies said, when the house band asked him to sing (as he’d done on his birthday last fall). This time, Butler obliged with hits by The Doors and Rolling Stones, as well as an “old-school freestyle rap.” He then brought the crowd to its feet with “Wonderwall.” “He looked the best he’s ever looked, and had the entire restaurant singing,” a spy said. “He killed it!”