but still no luck duplicating KFC
It's more about how they do it. You can find many copycat recipes on line. There is even one that claims to have the original written recipe. You can buy a spice blend from an online seller.
You start with a breading flour. For their original recipe, they add three packages. One is the spice blend itself, the next is salt and the third is a mixture of powdered milk and powdered eggs. Mix together.
The chicken is dipped in water then allowed to drain and then put in the breading tub. After the pieces are breaded, they are placed on shelves in a fryer basket.
Here is the part that they don't usually tell you about. The basket is placed in a pressure fryer, the lid is closed and the chicken is cooked under pressure.
After the chicken comes out and is allowed to drain, the chicken is placed on wire racks inside baking pans. These are placed in a warming box that has a pan of water for humidity at the bottom.
When they need chicken, a tray is removed from the warming box to a warmer on the order assembly line.
The extra crispy is a different seasoning mix. There is no egg/milk powder. After the first breading, the chicken pieces receive another dip in water and they get a second breading. The chicken is cooked in open fryers, not under pressure. This makes it extra crispy.
The main thing home cooks are missing is a pressure fryer. This cooks the chicken more quickly. The pressure helps the juices inside of the chicken and prevents the breading soft.