Cut into this flavourful cake and a cascade of sweets will come tumbling out – but don’t tell anyone beforehand and let the experience be a surprise.
Heat the oven to 180°C/160°C fan/Gas 4.
First, make the sponges. Pour the milk into a small pan and heat gently until just below boiling point. Remove from the heat, add the cocoa powder and stir to a thick paste.
Spoon the paste into a stand mixer fitted with the beater. Add the muscovado, baking spread and eggs, then beat on medium speed for 3–4 minutes, until fluffy.
In a separate bowl, add the chopped stem ginger and 1 tablespoon of the flour and turn to coat the ginger pieces evenly. Set aside.
Add the baking powder, remaining flour, ground ginger and orange zest to the egg mixture in the stand mixer and beat again on medium speed for 3–5 minutes, to combine. Remove the bowl from the mixer and fold in the coated stem ginger pieces.
Divide the mixture equally between the prepared tins and bake for 30–35 minutes, until risen and a skewer inserted into the centres comes out clean. Remove from the oven and leave to cool in the tins for 5–10 minutes, then turn out onto a wire rack to cool completely.
Meanwhile, make the buttercreams. For the American buttercream, beat the butter in the clean bowl of a stand mixer on medium speed for 3–4 minutes, until light and fluffy. Add the orange extract and icing sugar and beat on low speed until smooth and combined. Add the salt and beat to combine again. Transfer to a separate bowl and set aside.
For the chocolate buttercream, melt the chocolate chips in a bowl over a pan of simmering water. Remove from the heat and leave to cool for 10 minutes. Meanwhile, beat together the two butters on medium speed in the clean bowl of a stand mixer for 2 minutes. Add the icing sugar and beat on low speed until smooth. Add the cooled, melted chocolate and beat again until no streaks remain.
To assemble, using the biscuit cutter, cut a circle from the middle of two of the sponges to give two sponge rings. Reserve the cuttings. Spoon 5 tablespoons of the chocolate buttercream into a bowl and set aside. Divide the remainder into two portions of two thirds and one third.
Place one sponge ring on the cake board, using some of the larger portion of chocolate buttercream to secure it. Using the same portion, spread the top of the cake with a layer of chocolate buttercream (or, use an extra piping bag for this, if you prefer), then top with the second sponge ring.
Pour the shell-coated chocolate sweets into the hollow in the centre of the stacked sponges, then spread the uppermost sponge ring with chocolate buttercream, again from the two-thirds portion. Top with the remaining, whole sponge.
Cover the sides and top of the cake with the remaining chocolate buttercream from the larger portion, smoothing it over with a palette knife to give a neat crumb coat. Chill in the fridge for 15 minutes.
Break up the reserved cake cuttings into a bowl and use a fork to mix and mash them together with the reserved 5 tablespoons of chocolate buttercream until you have a dough-like consistency. Roll the mixture into approximately 20 balls of different sizes, then place them on the baking sheet lined with baking paper and chill for 30 minutes.
Remove the stacked cake from the fridge and use the smaller, one-third portion of chocolate buttercream to give it a second coat on the top and sides. Smooth it over until neat and chill for a further 15 minutes.
Divide the American buttercream equally into 4 separate bowls. Keep 1 bowl white and colour the remaining bowls red, green and dark green respectively. Into 1 small piping bag, put 1 heaped tablespoonful each of green, dark green and white buttercream. Spoon 1 heaped tablespoonful of green and white into another; and spoon red and white into the third. Fill the large piping bag with the remaining green, 1 tablespoonful of dark green and 1 tablespoonful of white buttercream.
Use the small piping bag with the two greens and white in it to pipe cacti around the side of the cake. Use the small piping bag with green and white to pipe smaller plants all around the base of the cake. Arrange the chilled cake balls on top of the sponge, to resemble stones.
Push the dried spaghetti strands into the top of the cake at varying heights. Carefully insert each strand through the nozzle of the large piping bag filled with the green, dark green and white buttercream and push down to the base. Pipe up each length of spaghetti to give upright cacti.
Using the small piping bag containing the red buttercream, pipe flowers up the upright cacti on top of the cake to finish the decoration. Serve in slices at room temperature, revealing the surprise in the middle when you do!