I've given you my suggestion for best hot chocolate in Rome and Florence so that leaves Venice.
From what I could gather through the various travel books and talking to folks who had been, there was no great place for sweets much less hot chocolate - shocking, because it would be a great place sure to thrive on the romantic aura of the city. Shame, but all was not lost. I actually found a wonderful cup of the liquid brown nirvana in:
Pizzaria Antico Panifico [Campiello Del Sol]
After walking over the Bridge of Sighs, we made a random left and stumbled upon this gem of a place looking for someplace to have lunch. It was perfect. Don't let the exterior fool you, I'd recommend this place in a heartbeat for more than the ciocolata. The pizza and pasta were to die for and I don't say that easily - not a fan of tomatoes or mushrooms, two things which tend to show up too frequently and in great quantity in American-style Italian food.
Inside this restaurant was a bright open feeling with a lunch crowd that was distinctly not touristy. You always hope to find those types of places and enjoy more of what they themselves like, not what a guide book says they like.
As for the ciocolata, it was thick and lovely.
There was no cream and that was fine, this was a cup of joy and needed no fussing. I find it interesting how, both in France and Italy, sugar is offered as if the hot chocolate needs it and I don't think it does. The flavor is perfectly balanced, the occassional dollop of cream is all that I personally want to stir in, but even it isn't necessary.
For me, Italy offered yummy ciocolata.
