The secret ingredient is love. (Or animal cruelty.)

ABC News:

Pssst. The secret's out at KFC. Well, sort of. Colonel Harland Sanders' handwritten recipe of 11 herbs and spices was to be removed Tuesday from safekeeping at KFC's corporate offices for the first time in decades. The temporary relocation is allowing KFC to revamp security around a yellowing sheet of paper that contains one of the country's most famous corporate secrets.

[...] So important is the 68-year-old concoction that coats the chain's Original Recipe chicken that only two company executives at any time have access to it. The company refuses to release their name or title, and it uses multiple suppliers who produce and blend the ingredients but know only a part of the entire contents.