Im not sure if this is the right forum to ask but oh well.
Can someone please tell me what mousseline buttercream is?
I read it under a picture on flickr but just cant find any good information
about it.

Like IMBC except the beaten egg white/sugar syrup mixture is added to the butter in the bottom of the mixer bowl.

This is my fav icing to use. It tastes sooo good. The recipe I use is from the Cake Bible.

Does this crust?

I don't think so, but I have never made it. If it is like my french buttercream,no. But I can decorate with it.