After developing a cult following for its ‘Shackburgers’, ‘flat-top’ hot dogs and eponymous shakes, the company is now preparing to serve up its own shares to the public through an initial public offering (IPO), sources said on Friday. Shake Shack’s majority owner, Union Square Hospitality Group LLC, has interviewed investment banks in recent weeks to appoint underwriters for the IPO, the people said. Shake Shack, which started out of a hot dog kiosk in New York’s Madison Square Park in 2004, is expected to post earnings of around $20 million next year, one of the people added.