I would torch the bolt (carefully) until it turns red.
I have found that rapid tap works very well for penetrating hot bolts, due to it's high smoke point.
just keep dripping rapid tap on the hot bolt until it quits smoking and starts penetrating. you will know its working when the smoke stops and the bubbles start, keep dropping the rapid tap until the bubbling stops.
let it cool back down to room temperature, don't touch it. It is important to let it completely cool down.
After its all the way cool, center punch the broken bolt (this is important)
Next, carefully drill straight down the center with a 1/8" left handed drill bit.
if it doesn't "grab" and pull the busted bolt, try an EZ-out,
...if that doesn't work
re apply heat, re apply rapid tap. let cool again. Move to the next biggest left-handed drill bit and repeat the process.
I used to be a boat mechanic, I have successfully extracted a bunch of busted bolts (and messed up a few too), This is the best method I have found. Go slow, be methodical, it'll come.
I can't overstate how effective rapid tap is for extracting bolts. That tip was given to me by an old mechanic and has proved to be the best lubricant I have ever seen for bolt extraction. ...and I've seen a lot of bolt extractions.
