broken exhaust stud removal

As trailbeast said, I use a carbide ball burr on a grinder to get a good center on uneven ends, sometimes I use a small wafer wheel to grind the edge flush, then put the divot in it.
I'm kind of the "specialist" wherever I work at this sort of thing, I flag .5hr/hole.....period. One trick that will work, You will need to make sure the head is reasonably clear of all
coolant & other flammables, is to drill up 'till the bolt/stud is 'thin".......then get a REAL torch w/a cutting tip and fire the hollowed broken fastener up 'till it's about to melt out.
Let it cool 'till it loses color, then quench it by spraying it with water. The thing will shrink up and loosen in there, may take a couple runs, but it will happen. You won't hurt the
head at all if you work fast & focus the heat on the fastener only, just used that one on the 352FE engine(late 50's Furd) in My tow truck, manny stud in blind hole.
Rust is hard, but it is also brittle, the heat can crush it & breakdown it's structure.
Oh.........and left hand drill bits w/a reversible drill................