Get a screw extractor at your local hardware store. It’s a drill bit with threads opposite the normal direction. You might have to drill out the busted shaft just a bit for the extractor to have something to bite into.
If you have enough post sticking out you can cut a groove with a dreamer. You’d then use a screwdriver just as normal.
You could try drilling into it then use a metal screw to grab it and back it out or break it whichever works best.
Couple of ideas.
You could use an extractor kit. Or if it’s not rusted in, take a dremel or file and cut a slot into it so you can remove it with a flathead screwdriver.
Source: Awning bolt snapped off, how do I best remove the post that’s still in there?
