Drill a small hole in them then use a screw remover with some heat.
I some time drill progressively larger holes in the broken bolt then at a certain point only the threads are left and they easily could be pulled out, but for this to work yo need to be centred in the broken bolt for the first small hole you drill, but very important do not drill into the threaded part.
Some times you need to use cobalt drill bits if the broken screw is made of a too had metal.
