Rubber like skin on surface of chocolate mousse When I make chocolate mousse, I usually end up with a slightly rubbery surface on the mousse.

To make the mousse, I do the following:


Melt dark chocolate in a bowl over simmering water
Separate 2 or 3 egg whites from the yolks
Whip the egg whites
Mix the yolks in with the chocolate and add a bit of sugar
Fold the egg whites in with the chocolate mix
Transfer to bowls and put in the fridge


Usually when I go to eat them the next day, there is a rubber like surface on the mousse, but underneath it's nice and soft.

I am just wondering what causes this, and how to prevent it?
The cause is that the mousse dries out on the surface. It builds a skin of dehydrated mousse. It happens with most cremes, not only your yolk foam. 

To prevent it, put plastic wrap on the mousse. Don't span it over the bowl, press it on the surface itself, without leaving air between the mousse and wrap. Your mousse will stay soft. This also works for custards and starch-thickened puddings. 

In general, also cover other things you put in the fridge, or they can get an unpleasant odour.