Danny Meyer is hungry to expand. The New York restaurateur is cooking up a $100 million IPO for his famed burger joint, where customers willingly wait hours in line. The public offering will allow America’s hottest burger chain to expand from its current 63 locations to at least 450 spots. Why all the hype? Fans rave about the hormone-free Angus beef on buttered buns slathered with a signature sauce. The challenge is to expand worldwide while keeping true to the quality-over-quantity approach.