The Lindt cafe in Sydney (in Martin Place) is rather disappointing. Not bad, but nothing like what Lindt/Sprungli do in Switzerland. Plus the place is jammed, so not much motivation to improve in the future.

The Chicago Pen chocolate buffet was mentioned on FT a couple of years ago and there have been several threads about it. However it is a hard thing to find when you are planning a visit to a city (and not something that most people look for, as it is so rare).