Once the ganache is on, refrigerate the cakes for another 30 minutes.
Insert some doweling supports for your cake into the larger 8" cake. I used four dowels. You want them to be towards the middle of the cake, for the 6" cake to sit on top of.
Using a small amount of buttercream, or ganache, spread that on top of where the dowels are, and stick the smaller cake on top of the larger cake.
If using, pour the edible paint into a small bowl and using a paint brush, flick the paint over the cake (make sure you protect the surfaces around the cake).
Add the the dark chocolate and milk chocolate to a large bowl. Add the cream to a pan and heat till just before boiling point. Pour over chocolate and leave to sit for 5 minutes. Once sat, mix until smooth (Add back to the pan if its not quite melted).
Using a small piping pag, drip the ganache down the sides of the two cakes, and fill in the tops of both cakes. If you can, let the cake set in the fridge for 30 minutes again.