With a perfect size sharpened punch a brittle hardened bolt extractor can be broken out crumb by crumb piece by piece. Takes an amazing amount of patience tho. I got in a manifold on a turbo once that took me all day on a front wheel drive car with no room to work a hammer. I broke a nice tiny snap on drill bit in one of the broken bolts prepping for the extractor.
Note a cheap Chinese metallurgy type punch not recommended lol.