Strawberry mousse with pistache cream inside, the Saya by Sadaharu Aoki is my favourite cake. Sadaharu Aoki have two patisseries in Paris, 35 Rue de Vaugirard near the Luxembourg park, and 56 Boulevard de Port Royal in the 5 arrondissement.
Very nice place where you can take your time for a coffee or eat french cooking by a Japanese chef. The design is like you're at home and there are different artist displayed on the walls.