My eldest son was mad about trucks generally, especially diggers and bulldozers, so my husband and I set about trying to make him a 'digger' cake for his 2nd birthday.
The base was easy, I just made my favourite caramel chocolate mudcake recipe and put it into a large square tin. I bought an icing mix from a cake shop which I had to add a heap of icing sugar to and then beat it with some green food colouring. You could also make up some butter icing yourself to use of course.
The digger was formed using butter cake. I made a packet butter cake and poured it into a rectangular tin. It was a bit fiddly but basically involved cutting the cake into three shapes, one for the base of the digger, one for the cabin, and one for the bulldozer section at the front. We assembled the base and cabin and iced all of the digger in yellow butter icing.
The struts joining the bulldozer section are banana lollies (trimmed to be fairly straight) with skewers pushed through them. We used licorice for the cabin windows and found licorice rolls at Woolworths for the wheels (again we pushed wooden skewers through them to help secure them).
I found the icing flowers and letters/numbers at Coles. We used marshmallows as 'rocks' that the Digger was pushing along. You could buy small ones but I think the normal size marshmallows taste better so we just cut those up. We added them just before serving as they go stale if left out.
This cake was time consuming to cut, ice and assemble because it was a first attempt and we had nothing to base it on. It was a huge success with my son and everyone at the party. We were told we should start a business!
My only tip with birthday cakes generally is to allow plenty of time as I always seem to be up until 1 am icing them! Worth the effort though. Good luck.