Preheat the oven to 180C/170C Fan/Gas 4. Grease and line two 20cm/8in springform cake tins with baking paper.
For the génoise, half fill a large saucepan with water and bring to a simmer. In a large heatproof mixing bowl, combine the eggs and sugar, then place the bowl over the saucepan (creating a bain-marie). Make sure the bottom of the bowl doesn’t touch the water.
Using a hand-held electric whisk, vigorously whisk the mixture for around 7 minutes, or until trebled in volume, pale and it leaves ribbons on the surface when the beater is lifted. Remove the mixture from the heat as soon as it is warm to the touch to avoid cooking the eggs.
Gently fold in the orange zest and salt. Sift the flour into the mixture in three batches, gently folding in each batch before adding the next. Slowly pour the melted butter down the side of the bowl and fold in.
Divide the mixture equally between the prepared cake tins, pouring from as low a height as possible. Place on the middle shelf of the oven and bake for 18-20 minutes, or until they are coming away from the sides of the tin and a cake skewer inserted in the middle comes out clean. Cool in the tin for 5 minutes then transfer to a wire rack to cool completely.
For the salted caramel sauce, put the butter and light brown sugar in a small saucepan over a low heat and stir until the sugar is melted (about 3 minutes). Drizzle in the double cream, mixing well – take care as the mix may splatter. Bring to the boil and simmer for 1 minute, then remove from the heat. Stir in the sea salt and set aside to cool, stirring frequently to prevent a crust forming. When cooled, whisk it for 1-2 minutes to incorporate some air.
For the cream base, whisk the double cream until stiff peaks form. Then whisk in the salted caramel to make a salted caramel cream. If the mixture is too loose, beat the butter until smooth and add to the salted caramel cream as necessary. Use a toothpick to add enough ivory food colouring to make a light brown/orange colour (if using).
To assemble the cake, cut each cake in half horizontally to make four layers. Carefully place the first layer on a cake board. Using a palette knife, spread about 5mm of cream on top in an even layer. Place the next layer of génoise on top and repeat the cream and cake layers until you have four layers of génoise with salted caramel cream on top of each layer.
Apply a ‘crumb coat’ of cream all over the cake, ensuring any gaps are filled to give a smooth surface. Make sure there are no overhanging edges as this will affect the mirror glaze later. Transfer the cake to the fridge for 20-30 minutes.
Remove the cake from the fridge and add a final layer of cream all over, finishing it carefully using a palette knife (the smoother the cream, the better the final finish will be). Place back in fridge for 20 minutes, along with any remaining cream.
For the chocolate glaze, add the cream, sugar, cocoa powder and 150ml/5fl oz water to a small saucepan and heat gently until the sugar is fully dissolved. Keep stirring during heating. Bring to the boil and simmer for 2 minutes. Remove from the heat and leave to cool for 10 minutes.
Soak the gelatine leaves in a bowl of cold water for 5 minutes. Squeeze any excess water from the leaves and stir into the glaze until fully dissolved. Use a sugar thermometer to check the temperature of the glaze and cool to a maximum of 38C before using.
Pass the glaze through a fine sieve into a measuring jug and tap on a hard surface to encourage any air bubbles to pop.
Once the cake has finished cooling in the fridge, transfer to a wire cooling rack placed on a large baking tray to catch any excess glaze as you pour it over the cake.
Pour the glaze evenly over the top of the cake. Check the sides of the cake are covered and add more glaze if needed by reusing any excess glaze caught in the baking tray. Immediately transfer the cake to the fridge to firm up.
To decorate, make a caramel. Put the sugar and 1-2 tablespoons water in a small pan over a medium heat. Do not stir, just swirl the pan. Heat until the sugar is an amber caramel colour, then remove from the heat.
Meanwhile, attach each hazelnut to the end of a co*cktail stick. Very carefully dip each hazelnut into the caramel (the caramel will be very hot, so take care not to burn yourself) and hang over the edge of a work surface, using a heavy chopping board to hold the co*cktail stick in place – protect the floor and any cupboards and with newspaper as the caramel will drip. This will form a caramel ‘spike’ as the caramel stretches. Once all hazelnuts are coated, trim the spikes to the same length using scissors and remove from the co*cktail sticks onto a sheet of parchment paper.
To finish, put the cake on a serving plate and pipe small circles of salted caramel cream around the top edge of the cake and place a caramelised hazelnut on each circle.
