The recipe is actually known. I came across a long article on the web about it. The true secret was that he used pressure cookers in the early years that no one else did.
Here's what Bing AI found and it might be from the article I read:
Here is an article I found on the KFC secret recipe. According to Joe Ledington, Colonel Sanders' nephew, the recipe consists of the following 11 herbs and spices: 2/3 tablespoon salt, 3 tablespoons white pepper, 1/2 tablespoon thyme, 1/2 tablespoon basil, 1/3 tablespoon oregano, 1 tablespoon celery salt, 1 tablespoon dried mustard, 4 tablespoons paprika, 1 tablespoon garlic salt and 1 tablespoon ground gingerš.
The recipe is one of the most valuable fast food formulas and is considered one of the biggest trade secrets in the worldÂł. The original handwritten recipe is housed in a 770-pound safe encased in two feet of concrete and guarded by video cameras and motion detectorsÂł.
This. All of the businesses who advertise (as opposed to withholding actual secrets) that they have "secrets" are doing this for marketing. Which is why places like Trader Joes are a gem: A plasticized label on one of their shelves says "Why do staff wear Hawaiian shirts and what's with the tiki theme? It's a marketing gimmick. When we started in 1969, we just thought it was cool. And we still do, which is why we haven't changed!"
Love how the AI gave a recipe with 10 items instead of 11, apparently Bing doesn't like black pepper since that's what it's missing compared to Wikipedia and the comment with the handwritten version
The true secret was that he used pressure cookers in the early years that no one else did.
Not a pressure cooker; a pressure fryer, which is a much more specialized piece of equipment. Unfortunately, you can't make authentic broasted chicken in your Instant Pot, and it's dangerous to try.
Usually it's a mix of salt and ground up celery seeds or sometimes dried celery, or occasionally lovage (which is a related vegetable.) It's pretty commonly available at grocery stores in (in the US at least) with the rest of the spices.
If you've ever had a Bloody Mary or Caesar it's a common (some would say essential) ingredient in them. It's also a major part of Old Bay seasoning.