ive had no luck getting broken valve cover bolts out. they are 1/4" and hard to drill in the center. did it break off flush?
what i sometimes do if i can't get it out is weld a old wrench or vice grips to the top and unthread it. if there isn't enough room to go all the way around with a wrench, i build up the weld on the top of the bolt and make it like a t and then use a small wrench or pliers.
with the valve covers, that would be my last choise because of the open springs and such. if you did, take a small damp towel and lay it over the springs and leave enough room to weld.