In "The Cask of Amontillado," to whom is Montresor telling this story?

Poe is the original master of suspense, and this short
story, "The Cask of Amontillado," is evidence of that fact.  The narrator talks to an
invisible and unknowable audience (us), but there is someone else to whom he speaks.  He
says:

The
thousand injuries of Fortunato I had borne as I best could, but when he ventured upon
insult I vowed revenge. You, who so well know the nature of my soul, will not suppose,
however, that I gave utterance to a threat. At length I would be
avenged....

These few lines
establish one clear fact:  the specific person to whom Montressor addresses these lines
is someone who knows him well.  We simply get to listen in on the story.  The fact that
he's telling this story fifty years after the fact suggests this is perhaps a letter or
some other kind of written work, which means it could have been just about anyone who
was at least somewhat familiar with the two main characters and this
setting. 

 To answer your question, then, we can't be sure
whom, exactly, Montressor is addressing.  We know it was someone who knew him well and
would, therefore, presumably not be as shocked as we are to hear about this outrageous
episode in Montressor's life.