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First, make the cake. Heat oven to 170C/150C fan/gas 3. Grease and line a 23cm loose bottomed cake tin with baking parchment. Put the chocolate in a heatproof bowl set over a pan of barely simmering water. Allow the chocolate to slowly melt, then set aside to cool a little.
Meanwhile, blitz the hazelnuts in a food processor until roughly chopped, then add the sugars and blitz again to a fine, sandy consistency. Tip into a mixing bowl and add the 1 whole egg and 5 egg yolks. This will be a very stiff batter, but with a bit of elbow grease, it will combine. When well incorporated, beat in the melted chocolate.
Put the remaining egg whites in a clean bowl and whisk until they form medium peaks. Scoop a quarter of the egg whites onto the batter and beat it in to incorporate, then gently fold in the rest. Pour the batter into the prepared tin and bake for 35-40 mins. A skewer inserted in the centre should come out reasonably clean, but the best indication that it is done is a smooth top with perhaps a few cracks, and a subtle springiness to the cake when gently prodded. Remove the cake from the oven and allow to cool completely in the tin.
To make the ganache, put the chocolate in a heatproof bowl. Put the cream and butter in a small saucepan and heat until the butter melts, but don’t let it boil. Pour it over the chocolate and leave for 1-2 mins to melt, then stir to form a smooth, glossy ganache. When the cake has cooled, remove from the tin and place it on a cake stand or platter. When the ganache has cooled to a spreadable consistency, spread generously over the entire surface of the cake – as neatly or as messily as desired.
To make the hazelnut spikes, place a large saucepan over a medium heat to get hot. Add the sugar and continue heating until dissolved. Increase heat and bubble, swirling the pan from time to time, until the caramel turns a gorgeous amber colour – but don’t stir as this will cause the caramel to crystallise. Remove the pan from the heat and allow the caramel to cool and thicken to the consistency of golden syrup – if it sets too solid, simply place it over a low heat to melt again.
Use a cocktail stick or wooden skewer to push into each hazelnut – make sure that you don’t push it in where the seam is or the hazelnut will split. Place a sheet of baking parchment on the floor directly underneath your work surface – this will catch any drops of caramel. Dredge the hazelnut through the thickened caramel, coating it very generously indeed. Secure the cocktail stick to the work surface with some sticky tape or wedge under a heavy pan, so that the caramel covered hazelnut hangs over the edge. Allow the caramel to drip, like a stalactite, towards the floor and set hard. Repeat with all the hazelnuts and allow the caramel to set. When hardened, use scissors to gently trim the fine threads of caramel off the more robust, decorative spikes. Arrange the hazelnuts, spikes pointing up, in the centre of the cake.