I've been trying to find out if white chocolate mud cake would taste nice with a dark chocolate ganache. This is for a cake that I am making for someone (so I can't make a mistake) and need to find out if this is a good flavor combination. I haven't been able to find out any example of such combination on the internet and was wondering why white chocolate mud cake is always paired with a white chocolate ganache and dark chocolate mud cake with a dark chocolate ganache.
I am assuming that by mud cake, you mean a tight crumbed chocolate layer cake, almost always iced with ganache.
There are a myriad recipes on the Internet for both regular chocolate, and white chocolate variants, which span the gamut of techniques and ingredients. While they do in fact most often call for a ganache matching the type of chocolate used in the layer, there are variations, such as strawberry frosted white chocolate mudcake, or coffee filled white chocolate mud cake. There is even at least one recipe for white chocolate mud cake with chocolate ganache.
I think you will find two preferences lead to the relative scarcity of white chocolate mud cakes with a chocolate ganache:
Nonetheless, there is no other reason not to pair a white chocolate layer with a chocolate ganache if you enjoy the combination; they should work well together, although the ganache will probably be dominant.
I suggest a practice run, perhaps with cup cakes, to test whether the combination works for you before trying it out for your friend.