Directions

1. Preheat oven to 180°C. Grease and line a 23cm cake tin.
2. Place the eggs and sugar in the bowl of a stand mixer fitted with a whisk attachment and beat for 20 minutes or until very light and fluffy. Sift the flour, cocoa powder and starch together then fold into the egg mixture gently. Pour into the cake tin and bake for 30 minutes until a toothpick inserted comes out clean. Allow to cool before unmoulding. Cut the sponge in half lengthwise.
3. For the crunchy base, gently melt the dark chocolate and butter together then stir in the crushed cornflakes and nuts. Press into the base of a greased 25cm cake tin and refrigerate to set. 
4. To make the mousse filling, heat the milk in a saucepan. Whisk the sugar, cornflour and egg yolks together then gradually pour in the hot milk. Return the mixture to the saucepan and cook until thick and glossy. Divide between two bowls. Stir the chopped dark chocolate into one bowl and the hazelnut paste into the other. Sprinkle the gelatine over the water and allow it to absorb. Microwave for 30 seconds until melted and then divide between the two bowls. Stir to combine. Allow to cool to room temperature.
5. Whip the cream until soft peaks form. Divide between the two bowls and gently fold in the cream. Place the sugar and water in a saucepan and heat until dissolved. Set aside.
6. To assemble the cake, cut a long strip of baking paper the same width as the cake tin and place it around the edges of the cake tin with the crunchy chocolate disc. Spread half of the chocolate mousse on the disc, then top with a sponge cake layer. Spoon 2-3 tbsp of the sugar syrup on top of the sponge then add all of the hazelnut mousse.
7. Top with a sponge cake and another 2-3 tbsp sugar syrup. Finish the cake with another layer of chocolate mousse and place in the freezer while you prepare the topping.
8. For the mirror glaze, in a saucepan, combine the water, sugar, fresh cream and cocoa powder. Simmer, for about 5 minutes, until the mixture slightly thickens. Soften the gelatine sheets in water and add to the hot glaze. Set over an ice bath and whisk until the glaze thickens and reaches about 26˚C, about 1 minute. Remove the cake from the freezer. Unmold the cake and remove the paper collar. Set the cake on a small cake tin on top of a baking tray. Pour the glaze over, all at once, so it runs down the sides. Refrigerate until ready to serve.