Edit to say I am not referring to Asm109
's post as half-assed! I hit post before I saw his spot on reply...
Half-assed solutions won't work here.
Pull the valve cover.
Get some cheap paper towels and cover the valve train, and push them down into the valleys against the head. Pay special attention around the broken bolt corner.
Spray the broken bolt with your choice of WD-40, PB Blaster, JB Penetrating Fluid, Kroil, etc. Use it liberally as you drill.
Get the smallest drill bit your comfortable with - I would start with a 3/32" and then use a 7/32" carefully, as it's getting close to the 1/4" diameter of the threaded hole.
Measure the depth of another bolt hole, and put a piece of electrical tape around the drill bit shank so you know when to stop.
Make sure you line yourself up perpendicular (90 degree) angle to the bolt/hole. Give the broken bolt a good dimple with a center punch to help the bit get a good start and not walk.
Drill carefully, not putting to much weight on the drill bit - let the bit do the work.
Repeat with the next size up.
If you get too close to the threads, a tap or even a thread chaser should be enough to clean them up.
It took more to type this than it takes to do it. Just take your time... then carefully pull the paper towels up with the shavings, vacuum the area if you need to get any of the shavings or dirt out of the head, and put it all back together.