In Paris, another vote for Angelina.
A Mont Blanc along with it??? That's overload for moi!
I usually buy the Mont Blanc on the way out at the bakery counter to have later.

At Cocoa et Chocolat on rue Buci is also good for a quickie. It's a chocolate shop, but they have a couple of seats.

In New York, the Union Square Cafe hot chocolate is excellent and they make their own marshmallows.