Foods/ingredients that are impossible to re-create
My list of these things:
Kentucky fried chicken (original recipe)
I have tried for years - decades actually, to make these and have decided it is impossible to replicate the taste.
I make and can vegetable juice every fall and it can never be quite as good as V8. It is good, but not AS GOOD.
Same with chicken - my fried chicken is tasty but for really great, I have to go to KFC.