If you hear a word Mont Blanc, you would probably think of the highest mountain of the Alps or the company Montblanc which is German manufacturing company of luxury goods.
However, in Japan is different. If you are not talking about Europe or mountain, Mont Blanc means a particular dessert in Japan. Actually it was created in 19th century Paris and therefore French people, especially Parisian know this dessert. However, it is not the same one which you encounter in Japan.
French original one is a dessert with sweetened chestnut puree in the form of vermicelli, topped with whipped cream. It is a dessert but not a cake. On the other hand, Japanese version is a cake, sponge cake or cup cake covered with whipped cream and topped with marron glace(candied chestnut) puree in the form of vermicelli and also with marron glace itself.
Japanese Mont Blanc is not a dessert served after meal, therefore hard to find in restaurants. On the other hand, it can be found at every confectionery store and at cafes. Mont Blanc is considered as one of the most popular cakes in Japan, especially for matured women.