The two answers to this flat differ by a single letter.

So what's inside my favorite candy bar?
The answer's simple, friend.
It's _ _ _ _ _ _  but  _ _ _ _ _ _ .

HINT:
Only one thing is inside the candy bar.

Answers:

It's NOUGHT but NOUGAT.

Credit: The Enigma
NPL Magazine - Jan. 2015
LEXMAN (adapted)