Believe me. / Believe you me.

Could anybody explain the difference of these expressions?

A: "Believe you me."
B: "Believe me."

Which one is correct in the following conversation?

"We would never find out what's wrong with it. ( )."
"OK, take it easy. Let's take time and figure it out."

Many thanks for your help.

There is no difference between the two expressions. "Believe you me" is, I think, a bit grammatically antiquated--but people still say it.

Maybe we only use it in the US. American English preserves many archaic features of the language.