Is this bolt threaded into the lower triple clamp ?
First off, can you tell if the bolt broke due to be overtightened, or from the bolt being seized due to corrosion ?
If the bolt is corroded into the triple clamp, do not use an extractor, you will just break it off and compound your problems .
First thing I would try, is to get a left hand twist drill bit of the correct size, try drilling the bolt to see if it may come out on it's own .
If not get penetrating oil on it, let it sit for a day or two and try drilling it again .
I would suggest taking it to a shop that has experience with this, as it can turn ugly real fast if you don't get it right the first time .