ASSEMBLING YOUR CAKE

To assemble the cake, cut each sponge horizontally into 2. Set one layer cut-side up on the cake board. Spread 4mm (¼in) of caramel cream over it. Place a second layer on top.

Repeat to sandwich together the 4 layers, making sure the cake is straight. Use just under half of the remaining caramel cream to completely cover the top and sides of the cake, making them flat and straight. Chill for 30 minutes. Cover the top and sides of the cake with the rest of the caramel cream (except the 4tbsp reserved for the decoration) using a palette knife. You may still be able to see the layers beneath. Chill the cake while you make the mirror-glaze. Put the cream, water, sugar and cocoa in a pan and heat gently, stirring constantly, until the sugar has dissolved. Bring to the boil and simmer, stirring, for 2 minutes. Pass through a fine sieve into a heatproof bowl and set aside for 10 minutes.

Meanwhile, soak the gelatine leaves in a bowl of cold water for 5 minutes. Squeeze the excess water from the leaves, then whisk them into the chocolate mixture until dissolved. Pass through a sieve into a jug and set aside until cold and thick but still fluid. Remove the cake from the fridge and place it on a wire rack set over a baking tin. Slowly pour the glaze over the cake to cover the top and sides. Chill until set.

To make the chocolate decoration, cut the acetate sheet into a few small strips of different sizes, somewhere around the size of 1cm x 10cm (½in x 4in) – make some a bit wider, some a bit longer – and cut lengths of masking tape, ready to use. Next, temper the chocolate for a shiny finish. In a heatproof bowl set over a pan of hot water, gently melt the chocolate chips. Place a cooking thermometer in the melted chips and slightly increase the heat under the pan of water, while stirring the chocolate. When the temperature hits 45°C remove the bowl from the pan and set it in a larger bowl of cold water to quickly cool. Gently stir until the temperature falls to 27°C. Set the bowl over the pan of hot water again and reheat, stirring, until it reaches 29-30°C, then remove the bowl from the pan.

Set the acetate strips on a worktop and spread each one with chocolate using a palette knife. One at a time, lift the strips up and curl them slightly, then stick the two ends together with masking tape to hold them in place. Put them on a sheet of parchment and leave in a cool place to set.

To make the caramelised hazelnuts, place the sugar and water in a pan and heat gently until the sugar has melted, swirling the pan from time to time. Push a cocktail stick into the base of each nut. Place the piece of foam at the edge of the worktop, jutting out, and secure it with a chopping board. When the sugar has melted, bring to the boil and cook rapidly, without stirring, until you have a chestnutcoloured caramel. Remove from the heat and dip a nut into the caramel (holding the stick), then push the end of the stick into the underside of the foam so the caramel drips down and sets to form a ‘spike’. (You may want to put something underneath to catch any drips.) Repeat with the other nuts.

Leave the hazelnuts until hard before trimming the ‘spikes’ to the same height with scissors. Pull out the cocktail sticks and set the caramelised nuts, spikes up, on a sheet of baking parchment. (You only need 8, but make more to allow for breakages.) Transfer the cake to a serving plate. Spoon two-thirds of the reserved 4tbsp caramel cream into the piping bag fitted with a star nozzle and pipe 8 swirls, evenly spaced, around the top edge of the cake. Set a caramelised nut on each, spike up. Put the rest of the caramel cream in a piping bag fitted with a small round nozzle and pipe beads around the base of the cake. Peel the chocolate ribbons from the acetate strips and arrange them in the centre of the cake.