Penetrability vs. blunt impact is an issue. For example, Kevlar vests will stop bullets because the fibers catch and spread out the impact such that pistol rounds won't penetrate. However, sharp instruments can slip between the fibers and vests need to be designed to be cut and stab proof. Vests for rounds come in different levels:
https://bulletproofzone.com/blogs/bu...t-vest-for-you
Now, to the issue - Buffy might have impact resistance for blunt blows depending on her flesh structure (if it is not mystical but physical) but not have penetration resistance. If she has a Kevlar head structure but bone isn't a fiber, might resist some handgun rounds. I am not up on her stabby resistance. If she is not stabby proof, kevlar structure and her skill is not like a ceramic, metal vest added plate, she is probably dead or severely injured with a good hit.
As far as the impact of handgun rounds, the standard today is that modern 38, 40, 357, 9 mm and 45 ACP rounds are pretty much the same with a good, significant area hit. Any of those can be designed with a round that will just go through bone. Those are hard cast lead for things like bear defense. Now, hollow points are designed to expand and not penetrate through you with a central mass hit but have enough penetration to reach vital organs.
You can move up the heavier handgun rounds, impractical for street carry, but used for hunting big animals. They would certainly smash through a skull that doesn't have some superhuman construction or mystical properties.
The lower powered rounds - it depends - a good 22 LR might penetration, the 25 ACPs might now - however, they have zapped into enough folks that there are no guarantees.
BTW, you probably don't want to aim at the forehead. That's thick bone and round do bounce off or zip around the skull under the flesh. You want to aim eyes to below the nose as to get back to the brain stem.
As far as other vital areas - they work by bleeding you out or severing the spine for a motion kill. Probably not a good idea with someone with superhuman resistance. Folks have taken 357s in the fight and stayed in the fight for a bit. She is fast enough to get you. If she then bleeds out, ain't doing you any good.