You certainly have heard about Angelina tearoom, famous for it's hot chocolate "l'Africain", and it's lovely pastries especially, signature "Mont Blanc" whose secret recipe remains closely guarded after one hundred years.
The décor, designed by famous Belle Epoque architect Edouard-Jean Niermans, is very elegant. The problem is that the place suffers from its own success. It's always crowded and you even have to queue to get in. That's why, I haven't been there for ages.
On the other hand, there is another Angelina tearoom where I love to go: it's the one at musée du Luxembourg. The coffee (and the little chocolate that comes with it) is just yummy. I often go there after a stroll in Jardin du Luxembourg.
My tip: in the summer, you can seat outside, at one of the outside tables.
19 rue de Vaugirard - 75006
Open every day. 10am until 7pm (and 9:30 on Mondays)