Broken cylinder head bolt removal
 

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.

There are 12 bolts that hold the valve cover on. The valve cover is sealed with an RTV like sealant (I used Permatex MotoSeal) The cover may just be stuck from the sealant and just needs to be gently pried off. Attached are two photos from when I resealed my valve cover so you can see which bolts hold it on.

The 3 that don't seem to do anything actually hold the rocker arm assembly in place.

Same happened to me
 

I was lucky my head bolt came out.

I used PB blaster soaked in hole. And at lunch today with punch and hammer it came loose gradually.
Threaded part was broke off.

As per the manual page 101 of 230 the 4 bolts that hold the head to the cylinder and bottom of engine are torqued to 27.5 foot pounds.
The cylinder bolts like 12a and others are torqued to only 7 foot pounds.
So it would make sense why they broke on us. I didn't use a torque wrench when mine broke, but definitely used more than 7 or 20 trying to tighten.