Here is a 5 inch four layer M&M cake. The cake is a vanilla cake, with vanilla buttercream. I loved making this miniature cake, and trying what worked best and what didn't. I don't have my cake turntable yet, so smoothing my buttercream took a bit of effort and lots of bending over, but I eventually got it. I used a 5 inch circular cookie cutter to make my layers from a sheet cake since I don't own any small cake pans.....yet (medical school budget).
Two things I want to remember:
1. Don't freeze your crumb covered cake for more than 15 minutes -- this will cause the rest of the buttercream you apply to go firm quickly, making it harder to smooth and stick sprinkles (or in this case mini M&M's) to the sides.
2. If using your own cardboard for a cake board (again, med school budget over here), make sure the aluminum you line with is smooth on the bottom so your cake layers remain smooth as well.
From this project, I have quite a few decorating items I now want to purchase to make this easier, but I definitely also learned that you can make it work with simple at home tools and a little imagination.