Use a sharp center punch to indent the center of the bolt.
Next use a very small Left Hand Drill Bit to slowly drill into the bolt (1/4" or so and do not forget to drill with the drill in reverse). Then move to a Spiral Flute Screw Extractor.

Irwin is just one company that offers screw extractor sets just for removing broken screws and bolts.

LH twist drill bit with CCW rotation of the drill (looking from BEHIND the drill) is the first thing to try for sure.

Rather than using a spiral flute screw extractor (EZ Out) use a flat cut concrete nail. They work as well as any screw extractor and are less prone to leading to a more complicated repair.

They are soft and ductile and will bend instead of SNAP like an EZ Out if you use too much force.

You haven't lived till you've had to drill out a hardened EZ Out that snapped off.