I did this for my son’s first birthday back in 2009.
I part-molded, part-carved two whole grain pound cake recipes. The exterior is buttercream and the filling is cream cheese frosting (of course!) and some fondant lox to visually break it up more.
Some days, I look at this and think I’d do the exterior in fondant if I had to do it again. Other days, I’m good with the rougher, buttercream finish.