12v exhaust manifold bolts seized and broke off flush with the head

I get in the rusted/broken bolt situation a lot at work and by far the best method I've found is to heat around the bolt and then while it's still cherry you quench it with water. It freezes the metal in the expanded state. I'm not sure i would do it on a head i planned on making a lot of power with though.

In your case i would heat the bolt, (not the area around it,) then let it cool back down to room temp. When it chills back down it will be slightly longer but narrower than how it started, allowing penetrating oil to do its job