My first action would be to drill a small hole down the centre of the bolt. You might need to centre punch it first to keep the drill bit in place. 3 or 4mm drill bit. About 5 or 6mm deep if you can.
Then find a torx screwdriver that nearly fits into the drilled hole but not quite. Tap it in with a hammer until the ridges grip, and if you are lucky (!) it will grip enough to unscrew the broken bolt.
My first action would be to drill a small hole down the centre of the bolt. You might need to centre punch it first to keep the drill bit in place. 3 or 4mm drill bit. About 5 or 6mm deep if you can. Then find a torx screwdriver that nearly fits into the drilled hole but not quite. Tap it in with a hammer until the ridges grip, and if you are lucky (!) it will grip enough to unscrew the broken bolt.
Good luck!
and heat the aluminum a bit as it expands faster than steel, might make it easier
