What was Montressor's idea of perfect revenge in "The Cask of Amontillado"?

In Poe's "Cask of Amontillado," Montressor's idea of
perfect revenge is confessing to us that he murdered Fortunato over fifty years ago and
knowing that he has gotten away with it and there's nothing anyone can do to punish him
for it.  In short, the idea worked as the perfect muder
and the perfect murder
story.

Not only was Montressor's plan
brilliant, luring Fortunato to his catacombs using a bottle of amontillado as a red
herring, but his execution of the plan shows bravado.  He foreshadows his vengeful
intentions to Fortunato several times on the passage down to the niche using the trowel,
the coat of arms, and much verbal irony.  But Fortunato is blinded and drunk by greed
and wine, playing right into Montressor's
trap.

Montressor's plan to bury Fortunato alive in an
underground vault also shows much forethought.  No one would think to look for him
there; much less, there could be little proof of a murder, since Fortunato's body is
walled up and mixed with other corpses' bones.

The real
brilliance of the revenge, though, comes in being able to retell it over fifty years
after.  Either Montressor is a brilliant mastermind or a lunatic who invented the whole
story.  Either way, as an old man, it takes a masterful mind to remember/invent such a
horror story knowing that he has impunity.