Combining chocolate with buttercream is NOT mousse, it's chocolate buttercream.
Real mousse is similar to what we Americans call pudding, although most mousses are lighter and firmer and richer (yes, all at the same time) than American puddings.
Traditionally, mousses are made with eggs, sugar, and flavoring, although many recipes also use heavy cream.
Here are my favorite mousse recipes:http://www.epicurious.com/reci.....sse-107437 http://www.epicurious.com/reci.....d_to_box=yhttp://www.foodnetwork.com/rec.....index.html
However, for cakes, I often just go with a whipped ganache. It is similar in taste and texture to mousse and much easier and faster to make.