What you're trying to do is make a "double dipped" fried chicken in a deep fryer. Can you do it? Yes.
Long story short:
Your oil temperature was too high. Knock it down 30*F to 40*F, or so.
A little nuance to the answer:
The trick with fried chicken is getting the coating, the skin, and the meat to finish cooking at the same time. There are some variables which prevent giving the one right temperature for all purposes, the most important of which are the type of frying and the size of the pieces.
Pan frying works at a slightly higher temperature than a deep fryer, everything else being equal. In my experience 350F, a temperature often recommended for both pan and deep frying, is slightly too high for either, while 325F, a temperature you sometimes see suggested for deep frying chicken, is slightly too low.
When you tested your already too-dark chicken and found the meat underdone, you failed to account for the "carry over" cooking which comes with resting. You're going to have to learn to rest and allow for carry over. Chicken is best after a rather extensive rest. And, in the case of fried chicken, is better warm than "piping hot." That the flesh was almost done when you tested was demonstrated by the fact that it was so overcooked with a few minutes more cooking.
Some ingredients and spices, if used in the flour dredge, will result in a very dark coating. Cinnamon and paprika are among them. So are bread crumbs and dried, green herbs.
A few "FWIW" thoughts:
A "chicken fryer" is a very common pan in the north as well as the south. The gourmet name is "saute pan." They're identical. "Alla time same same," as a young lady once told me. They're especially useful 12" and up. A 14" pan is large enough to fry a whole chicken's worth or pieces at one time.
There are a lot of good suggestions on this thread for cooking fried chicken in some other way than the one you've chosen. I've tried dozens of great ways, including all of the ones on this thread. That doesn't mean your way isn't just as good. Also, recipes don't come with shackles. If you find one way you like, you're not restricted to it for the rest of your life. If you want to perfect this one, then try something else; or if you simply want to move on -- you have my blessing, that's for sure.
Brining is a way of getting the meat to hold extra moisture. It is especially useful when frying at lower temperatures in that it gives you some leeway in that it allows you to cook the flesh more well done without drying it too much. Even then, poultry breast is has a very narrow range of done/juicy. Understand that I'm not suggesting that the way to fix your problem is through brining, but it may be of interest to you since it's such a natural variation you can do by simply oversalting the first buttermilk marinade. Of course you'll have to control salt by making a few changes. ^ou can't use the brined buttermilk between dredges. You'll have to use fresh buttermilk (or some other liquid) without any salt; and you'll want to reduce the salt in every other part of the recipe as well.
If you want to brown the coating to the desired color quickly at a relatively high oil temperature, then finish the chicken gently in a moderate oven -- it's a reliable technique, especially useful in quantities that require cooking in batches. Your crust won't be quite as crisp though. It's how I learned to cook fried chicken from Ora, the lady who used to "do" for my family.
I like fried chicken best with greens, corn bread, melon and sweet potato pie. That particular combination might be a Jewish thing. Probably originally Latvian or something.
Chalk's description of what the oil should look like during the cook was especially helpful, I thought.