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.
You enter La Fidelity and will find a very classic Parisian Brasserie, probably almost empty. Maybe one or two people at the bar. In the middle of this place you will find the stairs that go down to the cave. Have fun!