Clafouti is one of my favourite places in the city. They make croissants worth killing someone over. Their Pain au Chocolate in particular is absolutely amazing. The only problem with the shop is that their stuff sells out far too quickly. If you don't make it to the store before 10:00-11:00 you're probably not going to get a lot of the pastries they sell. Their croissant sandwiches are also great. This place is a real gem.
The shop is very small, so it's best suited for take out.