I was going to also mention that if you don't have a cake tin that you need, don't buy it until you have looked at hiring it. When I did the cake for DS's birthday, I hired a large 30cm x 30cm tin for $3.

Also if you line the base of your tin and the brush the sides with a bit of melted copha and dust it with rice flour, the cake comes away from the sides really really well. The cake tin was so clean doing it that way that I almost didn't need to clean it (I did clean it before I returned it )

I used a marbel of white and dark chocolate mudcake made without a packet and the recipe from The Crabapple Cupcake Bakery Cookbook for both. The dark chocolate marbel cake had coffee in it which I just substituted for milk instead and it worked fine. It did take over 4 hours to cook though and didn't burn. I then cut the large cake in half, put one half on top of the other and joined them together with chocolate ganache (that's how I got it so tall). Then I carved it and did a crumb coat (crumb coats seal in all the crumbs and make cakes much easier to ice - look it up on youtube).