Bake the cakes well in advance of assembling the cake. I initially thought I would do this whole thing on the same day, but it is easier not to.
Wrap the cooled cakes in cling wrap and either refrigerate or freeze until you are ready to assemble.
Make the Creamy Vanilla Buttercream as directed, but add in the cocoa. You may need to add a little hot water to the mix if it doesn't come together as the extra dryness of the cocoa can have an impact. Having said that, do persist and if you have a good quality cocoa (I use Cacao Barry Brut cocoa) it has a high fat content and it will be absolutely beautiful.
Slice both cakes into 3 layers. Spread a little of the buttercream onto a board or turning table and place the first cake on top. Spread with a little buttercream and then layer each cake on top until you have buttercream between all layers. Do a crumb coat with the remaining buttercream and place cake back into the fridge to set quite hard. This will take a couple of hours minimum.
Make the chocolate ganache in advance of being ready to finish the cake. We suggest dark chocolate gives the correct look as opposed to milk chocolate.
Spread and pour the ganache over the cake before it has fully set. Then use a palette knife to do your best to get an even coating. You can see I had trouble, but I have added all my hints here for a better result. (For example I didn't freeze or even refrigerate my cake prior to assembling and that would have gone a long way to a better shaped cake.)
Allow the ganache to set fully before trying the cut the cake. It is a sticky ganache for taste and look purposes, so it does melt quite quickly at room temperature. Keep refrigerated between slivers. We are eating it a week later and it just gets better and better.