Add your favorite brand of PENETRATING oil (Not WD-40) and allow it to soak over-night.
Use a spring-loaded center punch and mark the dead center of that broken bolt.
Heat the area surrounding the threads with Map gas until nice and toasty without setting fire to your shop.
Allow it to cool with more PENETRATING oil applied after it no longer bursts into flames.
Drill it out using LEFT-HAND drill bits. Starting with about 1/8" and work your way up in size until it unscrews itself.
Report back on your success.
Eric