For the best fondant chocolat cake in Tokyo, go to Ginza Manaya 銀座まなや between Shinbashi and Ginza. A refined, modern but traditional Japanese-style izakaya, they have the best fondant chocolat ever. And the kara-age is divine. The roast beef is a specialty, but normally only available at lunch. If you ask nicely, they might be able to accommodate you with a small plate at dinner though.
For mango shaved ice so finely shaved that it's like snow, go to the Taiwan Products Shop. Hidden to the right back side of the store is a little cafe where you can get traditional Taiwanese tea foods and the best mango shaved ice ever.