A friend of mine just asked for some tips on how to make a Barbie princess cake so I thought I would copy my reply to her here for anyone else who might be interested. I made this for my daughter's 4th birthday last year.
There are several ways that you can make the doll cake:
1. You can buy doll's head and torso from most cake decorating shops (made by Wilton) but personally I don't think they look that nice.
2. You can use a Barbie doll and either bake a lot of cakes to stack to the height of the doll, or you can take the legs off the Barbie (most seem to come off quite easily) so that you don't need to make it so high.
I found the following video clip really useful http://www.howdini.com/howdini-video-7075975.html.
I opted for the full height Barbie and made a selection of cakes - 3 medium width ones, 3 small width ones and one shallow one baked in a pyrex bowl to give the curve at the top. Another suggestion for the curve at the top is to use a doughnut or a bagel. I stacked them on top of each other and carved the outside of the stack to give a smooth slope. I think it looks like a dalek at this stage! I then cut a hole out of the middle of each one and re-stacked them with buttercream in between to hold them together. A trick I learnt on my cake decorating course, which would have been helpful, is to align cocktail sticks in each layer before separating for the buttercream. It is then easy to re-align the cakes in the position they were cut.
I used fondant to cover the cake (over buttercream) and had a front panel and then a skirt draped around the back and onto the front panel. That was tricky as it needed a huge amount of fondant to cover the sides and back - much more than you think. It would be significantly easier to cover the whole cake in coloured buttercream (see the video attached) but I just like using fondant.
I also made the bodice in fondant which was a bit fiddly and in hindsight I could have dressed the doll in normal Barbie clothes and then just covered the join with a ribbon (as I did) or fondant flowers etc.
A useful tip is to wrap the Barbie's legs in clingfilm before you put them into the cake so that they don't get covered in buttercream in the middle of the cake. Also my holes in the middle of the cakes were a little too big so I dropped a few cake off-cuts into the hole and pushed the Barbie into these to keep her steady.