Broken cylinder head bolt removal

[buud]

The left front cylinder head bolt ( 12A ) broke when I was torquing it tight if stop oil leak. I got to 20 pounds on the way to 27.5 pounds when it broke.

Couldn't extract the "1/2 inch" bolt stub still in the head so I've turned to removing the valve cover in hopes of seeing a bolt stub protrusion.

Removed 12 bolts ( and loosened three others that apparently have nothing to do with holding the valve cover on) and tired of whacking the living daylights out of the cover.
I used a wooden dead blow mallet and wailled away with no success.

Its on there so tight, its hard to understand why it was leaking in the first place.

When struck from above there seems to be a slight "jiggle" sound toward the front but solid silence from the rear part. Striking on the sides or from a upward direction where a bight can be obtained, yields only unmoving metal sound.

Can't figure what I'm doing, or not doing, wrong.
Any advice is welcome and appreciated.