Mont Blanc is a French dessert named after the tallest mountain in the Alps. It's sort of a flavored meringue drop. The Japanese transcription is モンブラン or "monburan".

[EDIT] As for the joke of the commercial, I think it's just that the man is walking in to the factory to begin making the Mont Blant and Petit Chocolat, and suddenly a celebrity (Ayu) walks out from the doors, already eating the snacks directly from the box. Kind of subtle and muted, but I think it's more cute than funny.