Trim the tops of the cakes to level them. Place one of the cake rounds on a flat board (use a little bit of buttercream to stick it in place) and place that on an icing turntable.
Spread a little of the white buttercream over the cake and place another cake round on top. Spread over some more buttercream then place the final cake layer on top upside-down so that the top of the cake is perfectly flat.
Spread a very thin layer of the white buttercream all over the cake (a crumb coat) then place it in the fridge for 15 minutes.
Transfer the rest of the white buttercream to a disposable piping bag and the black buttercream to a separate one. Snip off the tips so that there is an approx 1-1.5cm hole.
Return the cake to the turntable and pipe a ring of buttercream around the base. Pipe a ring of the opposite colour on top of that and repeat, alternating colours, until you reach the top. Fill in the top of the cake with whichever colour you have most of left (black in my case). (See image above)
Use a bench scraper or icing spatula to smooth the top of the cake. Don't worry if it's not totally neat yet.
Hold the edge of the bench scraper vertically against the side of the cake. Spin the cake slowly while you hold the bench scraper steady and smooth the buttercream. Stop every few turns to scrape the excess buttercream into a bowl. Wipe the bench scraper down with a warm paper towel to clean the blade between spins.
Use the flat edge of an offset spatula to clean up the top edge of the cake. Swipe the blade across the top of the cake, pulling in from the sides towards the center of the cake. Do this all the way around until your cake is smooth enough.
Pop the cake in the fridge while you make the ganache drip. Place the chocolate and cream in a bowl set over a pan of gently simmering water and heat, stirring regularly, until melted and smooth. Remove from the heat and stir in some black gel colouring. Set aside to cool a little.
Carefully spoon the ganache around the top edge of the cake, using the back of the spoon to encourage it to drip over the edge. You can go all the way around the top if you like, or do as I did and just cover half of the cake.
Top the cake with your selection of sweets and serve. The cake will keep well in an airtight container but don't add the sweets until just before serving as they will moisten a little over time.