Please help... does anyone have a stable chocolate mousse cake filling.

My granddaughter has requested a princess cake with vanilla cake and chocolate mousse filling.  However, I cannot find a mousse that is stable enough.

I have tried everything and it does not hold up well.  It either melts and comes out the sides of the cake or I have to put a very thin layer which I don't want to do.... cake to filling ratio.

At this point I am willing to buy it but I do not have any place in my area that sells it.

The cake will be covered in fondant and sitting out for a few hours.  Also, can I make the cake ahead and refridgerate it without the fondant getting condensation all over it.

Thanks in advance. 

Mousse should not be standing outside for several hours in a cake.  like cream cheese frosting it needs to be chilled.  Have you chilled your mousse first with plastic totally covering it so no skin  and then piped it as filling?  Again it needs to stay chilled. Also a buttercream dam around edges to hold mousse in.

Try a whipped ganache. For the vanilla cake I would err on the side of combining white chocolate and milk chocolate to make your ganache. Mix up a 2.5 part chocolate to 1 part cream ratio ganache, let it set overnight on the bench then whip it up - yummy, not quite as fluffy as a real mousse but a really good compromise for what you need.

Consider a vegan chocolate mousse made with whipped coconut milk. Since it is non dairy, refrigeration is less critical.  

sewsugarqueen is right -- all that stuff, dairy or non-dairy has to be kept cold -- it starts to deteriorate -- whipped ganache like bubs said is a great alternative -- does not require refrigeration --

and you could always make the cake vanilla and for serving, plop a big scoop of mousse on each slice -- mousse that you've kept cold in the fridge the whole time 

The "mousse on the side" thing is the best bet. That way, you can make the cake the way you like (shelf stable), and not worry about poisoning people with spoiled cream. Make up those huge piping bags filled with the mousse/ cream and keep in a cooler until serving time. Serve cake with a 1M swirl of mousse on the side .... then you had presented the cake well, and served it "fancy". Depending on the age of the granddaughter, I might add sprinkles on the mousse (another big hit with kids). WIN/ WIN. 

 piped! yes brilliant, johson6ofus --

you could even make smaller bags and let her pipe some if you want to clean it up 
win win mess win hahahaha

You can make up some SMB and add melted chocolate to it.  Whenever I do this people think it is a mousse filling.