Place one of your cooled cakes onto a cutting board. Using a small serrated knife, lightly mark out a smaller circle on top of the cake, about an inch in from the edge on all sides (I used a large tea cup as a guide- a small saucer or 4-inch cake tin would also work well). Once you have the inner circle marked out, use the same serrated knife to carve off a bevelled edge all around the cake (see photo in post and Note 5). The aim here is to mimic the rounded shape of a carrot top, but don't worry about being too precise, because this will be covered in icing later on!
Place the offcuts of cake from the previous step into the bowl of a food processor, along with the bourbon biscuits (I used a mini processor for this, because it was such a small quantity). Blitz until you have a fine crumb, then set aside.
Set your uncarved cake on a serving plate. Place 1/5th of the cream cheese icing on top and spread out. Carefully place your carved cake on top, with the flat edge facing down. Taking about 1/3rd of the remaining icing, cover the whole cake in a light crumb coat, following the bevelled edges along the top. Once done, place into the fridge for 15-20 minutes, until the buttercream has formed a crust.
After the crumb-coated cake has chilled, take it out of the fridge and ice liberally using the remaining icing. Take your time smoothing out the sides and rounding out the bevelled edge. When you are happy with the icing, start positioning your white chocolate shards on top of the cake, in the centre, varying heights and directions. You may want to use a knife to create a small incision for each shard, instead of trying to force the chocolate in, which may break the thin shards. Fill in any gaps with smaller shards and splinters of the chocolate.
Finally, use a teaspoon to scatter the cake crumb/biscuit 'soil' all around the base of the cake, pressing it up the sides a little as you go.