The process is quite simple. This is a double cake, filled, and I´ve used foundant icing to create the shape of a real brain. Then, just colorized with food color spray.
1- Start by creating two cakes, with circular shape.
2- Fill the cakes what each flavor you want. This will make the cake more resistant to cuts that we gonna do, and make it more tasty of course.
3- Using a knife, make a longitudinal wedge on top and continue along front and back.
4- I´ve used foundant icing. You can get the recipe on various websites or in Youtube, like this one: http://www.youtube.com/watch?v=ZU-PpjZQjLI
5- After place the foundant icing, use a scoop or a paletteknife to sculpt the brain nodules.
6- Finally, use the color food spray. First the red, spread it equally. Then you can use Blue or Purple to create a tumor impression, or the Red again on parts of the surface, to be more irregular colored.
7- I didn´t took a photo, but the when you have done, sticky a knife on it.