You can purchase a 1/2 or 9/16 Cobalt drill bit to drill through the outer shoulder, or the tapered head. The secret to drilling any material is watching for the long curly shards. If you are not getting the curly strands of metal than the bit is spinning too fast, slow it down until you see the curls consistently and stay with that speed. The Milwaukee 1/2 variable speed drill is my tool of choice; I can turn the bit slow enough to cut through that hardened bolt. Plan on 10 to 15 minutes for each broken bolt and try to stay as centered as possible when drilling.

When drilling metal keep in mind "if you hear the bit squealing you have no metal peeling!"