When you break off a bolt, don't panic. For those of you who don't live in the rust belt, this might not appy. EZ outs don't always work and if they break off your in big trouble! Dad tought me this he was a tech. for 43 years. No he didn't invent it.
Take a drill alot smaller then the bolt and drill a little off center, but not to much. Keep advancing the size until you usually see rust. This means you have touched the threads, STOP. Now take a punch and start folding the bolt inwards to distort it. If you can access both sides this will speed things up. Never ran into one that this didn't work if you take your time.
Sorry I forgot about the thread chaser. To use to doing it each time. Good catch TomO. I like the offset because it gives me something to hit,works best with a little bolt above the surface. Both ways will work.