For this cake, I stacked up six layers of white cake with vanilla buttercream and covered the outside with ganache- the recipes for these are the same as the ones I used for the Creme Egg Cake. I then simply broke up a lot of regular-sized KitKats (about 2 packs) and arranged them around the outside, and poured M&Ms over the top.
Tragically, I was about one Kit Kat away from being able to cover the entire circumference (no one seemed to notice the difference... after all, how could you criticise a cake slathered in ganache, laden with Kit Kats and M&Ms and dusted with copious amounts of glitter?).
The Kit Kats comfortably stayed in place because the ganache wasn't too thin but you could also tie a ribbon around the outside for extra support and decoration.