Use a dot punch to carefully tap the broken off part anti-clockwise. You will need to tap a dimple on the outer edge of the exposed end, then tilt the punch and tap with a small hammer at an angle to rotate out the broken off part.

Chris

 

I feel your pain, I did the exact same thing except to the bolt holding the cam chain tensioner, probably the most frustrating feeling on this earth. The car ended up sitting out of commission for about three days as I tried a couple different approaches to get the damn thing out. first I tried the simplest solution which was grinding a slot in the bolt with a dremel and then screwing the thing out, no joy. What finally worked was a healthy dose of break-free and then a set of left hand drill bits or broken bolt extractors hooked up to a little mini hand drill. You only really get one good shot at getting the extractor to grab the bolt so make sure u make a nice deep punch right in the middle of the bolt and then really put some pressure on the extractor as you begin to rotate it.

The feeling you get once you first see that thing start to slowly creep out is one of pure joy.

If you need to find a replacement bolt, I found an exact replacement at an Ace hardware store.

Good luck