Who is the ¨you¨ addressed in paragraph 1 in "Cask of Amontillado"?

The line that you are referring to is, "You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat."

The story is told in the first person by Montresor. The "you" is not specifically mentioned during the story. Perhaps who "you" is, can be understood if you consider the last lines of the story, "In pace requiescat!"

The last lines mean rest in peace in Latin. At the end of the story, Montresor mentions that no one in half a century has disturbed where he placed Fortunato.

The "you" may be the reader, Montresor, or God. The story could be a form of confession- a way for Montresor to make amends with himself and God for what he did. I mention God because there is an exchange between Montresor and Fortunato toward the end of the story involving the phrase, for the love of God. After Fortunato exclaims these words, Montresor says, "yes" and also exclaims, for the love of God. 

Asking why Montresor wrote the story may lead to who the "you" may be in the first paragraph.