OK. I understand that "chocolate" is not a restaurant. But, if you go to Lausanne, you must eat chocolate.
There are literally hundreds of chocolate shops in Lausanne.
My personal favorite (because of proximity to where I lived) is Noz Chocolatier. Located on a little side street (Rue Marterey) not far from the Bessieres metro stop.
Address: Rue Marterey 11, 1005 Lausanne, Switzerland