The portions still threaded into the head are likely quite loose. However, If you try to drill to drill them out, the drill will spin them down to the bottom of the threads. This will jam the bolts pretty tightly, because the tap that created the threads in the head is tapered. So your bolt spins down and jams in the taper of the threads. The best solution is to get a left-handed drill bit to drill a hole in the broken bolts. Very likely they will spin right out in the drilling process. If not, and especially if you used a chemical threadlocker (loctite), apply some heat (which loosens the threadlocker) and try an easy out. Broken bolts require patience and a determination to not cut corners while making your first attempt at removal. The first attempt can often be your last without having to do a repair to fix the mangled part, so make your first attempt the best you can. If you can't buy an appropriate left hand high speed steel bit locally, order on line and be patient!
Takes a bit of practice to be able to correctly judge how tight to make any fastener. Torque wrenches are as often used to tighten very small fasteners to avoid over tightening as they are to tighten high strength applications such as head bolts to a minimum and even value. Get one and practice! Those slipper bolts you broke off probably only are supposed to get 7 to 10 foot/lbs of torque, which you can deliver holding just the head of a 1/4 inch ratchet without straining! (totally guessing at the torque value so look it up!).
On the bright side, you broke the bolts as opposed to stripping the aluminum! You still have a good chance to fix it without having to do any machine work !