Fudge as a cake layer?
Fudge icing is actually not made with fudge believe it or not, often it's a chocolate buttercream but there are other recipes. There are a few problems I can see with using traditional cooked fudge as a cake layer:


It is not the right texture for a cake: Imagine you're eating nice fluffy cake and then you run into hard, gummy fudge which welds your mouth shut
Fudge would be very hard to work with. You cannot spread it, you'd have to roll it out which would be messy
Traditional fudge would not keep the layers together. Part of the role of the filling is to stick one cake layer to another so that they stay put, and pieces stay together when cut and put on a plate. A cake with a hard fudge filling would would be hard to decorate as any sideways pressure on the top cake layer would make it slide, and when cut it would fall apart.