That's pretty odd. Usually when such a bolt no longer has any pressure on it (from being tightened down) it will just unscrew easily by finger or with pliers on the exposed end.
If I recall, without looking, doesn't that bolt thread into the aluminum head? The torque used on that bolt is only about 25 ft/lbs, how could you have possibly tightened it enough to snap it!? I would think the threads in the head would strip before the steel bolt would snap.