How did it break off? Drill the center out and hammer an ez out in the extract it...easier said than done sometimes I have an intake bolt stuck in my gt40 heads and I'm still working on it without destroying the threads.
if it broke off high enough, you should be able to just grab it and unscrew it. there wont be any torque left on it, so it should come out easy. if its below the top of the threaded hole, you will have to use the ez-out like mentioned above.