Here are the five ingredients I use to make Chocolate Orange Mousse. The ingredients list can be reduced to just two of these if you want to make a no frills chocolate mousse.
As you can see, there is no cream and no sugar. It's positively diet fare.
The ingredient list may be small, but disaster can strike at any moment with chocolate mousse, and once it's struck, there's no recovering and you just have to start again. It is all too easy to overheat the chocolate when you are melting it, or get water in it, or stir it too much or too vigorously, making it go grainy and preventing it from incorporating well with the egg white. It is also all to easy to overbeat the egg white, breaking up the protein strands so the egg white goes dull, separates a bit and doesn't hold air. And you have to be ever so gentle folding the beaten egg white into the melted chocolate, and time it just right so the chocolate is not so cool as to have started to reharden, but is not still warm enough to cook the egg white or cause the bubble matrix to collapse.