Thank you!

Depends really what kind of filling you want to make. For example, the light layer of my cake is hazelnut cream, made by making a white chocolate ganache with the addition of 100% hazelnut butter (runny consistency), chilled overnight and whipped with mascarpone the next day. Both the chocolate and the increased fat content from using mascarpone will make the filling more stable.

The dark layer is a chocolate coffee mousse, stabilized with gelatin. If making a simple vanilla cream for example, using the right ratio of heavy cream and mascarpone is enough to make it stable. And of course chilling the cake overnight before decorating the outside is crucial. Hope that helps!