I want to tell you a little secret not many people know about: there's a small little cafe near the Sorbonne University where you can drink a heavenly hot chocolate made with pure dark chocolate and topped with the best whipped cream in the world :) Trust me, once you've tried this hot chocolate, you won't want any other...
[photo taken from here]
[photo taken from here]
The last time I went there was in 2007. As you can see, though my arm was crushed by my sleeping baby, I was in chocolate heaven.
The cafe is really very small and you might have to wait to get a seat but it's really worth it. They're closed on weekends so be sure to make some time during the week to come here (maybe when you visit the Latin Quarter or St Germain which are really close by).