This pre-prohibition classic from the famed Waldorf Hotel (before it became the Waldorf Astoria) was recommended to me by my old friend Joe Adams. There are a lot of words online about it, mostly trying to adapt it to "modern tastes", as if today's cocktail fans would be too challenged by the absinthe.
There's some pretty bad absinthe out there; use a good one. Absinthe has a strong flavor, so a strong rye and strong sweet vermouth are also important. I used a cask-aged sweet vermouth, but Carpano Antica would go well.
As for the rye, I started with Old Overholt, but once I had a good formula I switched to 100-proof Whistle Pig... Now there's a cocktail worthy of the Waldorf!