I have to make a 4 wheeler cake with mud on top of it, and the customer likes the look of icing on the top of dirt cakes (crushed oreos and pudding). I'm not sure how to do this on a cake and it taste good because the sides of the cake are going to be buttercream. Should I use ganache? If so, do I put a layer of buttercream under the ganache? Any help is greatly appreciated!

Here's a link to a cake that was posted 2 days ago. It shows several
views. HTH
http://www.cakecentral.com/cake-photo_1756052.html

Here is a 4-wheeler cake I did. All buttercream with graham cracker crumbs sprinkled on.
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1646332

I found that using my hands to "smooth" and "texture" the buttercream was the easiest.

You could make two different shades of chocolate and mix those together as well.

Good Luck!

I would think you could base coat the whole thing in bc, then go over the area you want to decorate with the oreo/pudding mixture. Ganache would also make great mud though. Personally, I would ice the cake first like normal and do the mud over it.