This moist chocolate cake is surrounded by crunchy chocolate soil and decorated with a piped garden. Feel free to be creative – take the opportunity to design your own garden!
Heat the oven to 180°C/160°C fan/350°F/Gas 4.
Make the sponge. Put the cocoa and boiling water in the bowl of a stand mixer fitted with the beater. Beat together, then add the remaining cake ingredients, except the colouring, and mix at medium speed to a soft batter.
Divide the mixture between the prepared tins so that the depths of the cakes are equal. Bake for 25–30 minutes, or until a skewer inserted into the centres comes out clean. Leave the cakes to cool in the tins for 5 minutes, then turn out onto a wire rack to cool completely.
Meanwhile, make the Italian meringue buttercream. Place the caster sugar and 100ml of water in a medium pan and over a medium heat. Heat gently until the sugar has dissolved, then increase the heat and boil until the syrup reaches about 110°C on the sugar thermometer.
Whisk the egg whites in the clean bowl of a stand mixer to soft peaks. Keep the whisk running and, when the sugar syrup has reached 121°C, remove the pan from the heat and slowly pour the syrup in a thin, steady stream into the whisking egg whites.
Continue whisking until the outside of the bowl is at room temperature and the meringue is very thick and glossy.
When the mixture is at room temperature, start adding cubes of salted and unsalted butter, whisking after each addition to make a smooth and thick buttercream. Add the orange, vanilla and lemon extracts and mix thoroughly. Cover, and chill until firm.
Make the chocolate soil. Heat 2 tablespoons of water and the sugar in a small pan over a medium heat until the sugar dissolves. Increase the heat and boil the mixture until it just starts to turn golden around the edges – about 135°C on the sugar thermometer.
Stir in the chopped chocolate – it will immediately form clumps and crumbs. Tip onto the lined plate and leave to cool.
To assemble, sandwich the 2 larger cakes with 3 tablespoons of the buttercream, spread evenly, and place on a plate. Spread 1 tablespoon of buttercream on top.
Sandwich the 2 smaller cakes with about 2 tablespoons of buttercream and stack these on top of the larger cakes.
Place 6 tablespoons of buttercream in a small bowl and add green food colouring to make a leaf colour. Place 4 tablespoons of plain buttercream in a second bowl and colour this flower-like red.
Spread the remaining plain buttercream around the top and sides of the stacked cake.
Place the red and green icings in separate small disposable piping bags. Snip off the tip of the red icing and make a V-shape cut in the end of the bag of green icing.
Pipe green leaves around the sides of both cake layers and a few on the top, adding red dots to make flowers. Sprinkle the chocolate soil around the bottom of each cake layer and sprinkle a little on top. Chill the cake and serve within 2 days.