Cut the domes off of the two chocolate sponges so they have flat tops
Cut each cake in half
Place one of the cake layers down on a plate or cake board and spread with chocolate buttercream, add another layer on top followed by more buttercream
Before adding a third cake layer, pour some of the chocolate ganache on top of the buttercream and spread to the sides (don't add too much or the cake will be slippy. Add the third cake layer on top, followed by more buttercream and the final cake layer
Once the cake is stacked, cover the whole thing in chocolate buttercream and either smooth it out. Put the cake in the fridge to chill for 15 minutes
Once the cake has chilled, heat the chocolate ganache in the microwave for 5 seconds, just to soften it up a little, put it in a piping bag and cut a small piece off the tip
Use the piping bag to pipe drips of ganache down the side of the cake, all the way around. Once you have all your drips, use the remaining ganache to cover the top of the cake, smooth it out and place it back in the fridge for 5 minutes
Take any remaining chocolate buttercream and put it into a piping bag with a star nozzle, pipe rosettes on top of the cake and finish with some chocolate sprinkles
Notes
Cut the two cake layers in half length ways to make 4 layers total, and frost in between each layer with the chocolate frosting. Then coat with the chocolate ganache.