I used 8-inch rounds of chocolate chocolate chip cake, frosted with dark chocolate buttercream. The top white portion is made from marshmallow fondant (although I really should have paid more attention to the game play because it's really only white when the game is off; otherwise, it constantly changes color).
The rock walls are chocolate clay. I was really happy with how easy it was to make the rocks from chocolate clay. I've only ever made roses with this stuff before, so I made sure to make the recipe a little stiffer so it wouldn't slump. It worked beautifully and tasted like fudge. Also, it was still easy to detail so it had the right textures and symbols.