Did you mean that an extractor broke rather than a tap? I am asking because you would normally remove the old bolt before you would retap the hole. In any event, both are very hard metal and will be a challenge to drill out but that's what you would need to do. Pick up several good quality drill bits of the appropriate size and use cutting oil to help with the drilling. If you make the hole too big, you may have to pick up a helicoil kit to make the repair.