Prepare chocolate sponge cake. Preheat oven to 325F (160C). Grease with butter a 8 inch (20cm) pan and line with parchment paper.
Separate whites from yolks. Add salt over the whites and start whipping until foamy.
Gradually add sugar and continue whipping until stiff peaks form.
Gently fold in beaten egg yolks, ground hazelnuts and cocoa powder.
Pour the batter into the prepared pan and bake for 15-20 minutes. Don’t overbake.
Prepare crunchy hazelnut streusel. First prepare the streusel dough. Keep the oven preheated to 325 F (160C) . Mix butter with sugar and salt until combined.
Incorporate hazelnuts and flour until homogenous.
Roll the dough between two sheets of parchment paper or plastic wrap until thin. Refrigerate for 30 minutes before baking.
Bake for 20-25 minutes or until golden. Let it cool completely.
Break into pieces and turn into crumbs.
Transfer the crumbs into a bowl and mix with paillete feuilletine.
Gently incorporate nutella.
Cut the sponge into a 7 inch (18cm) disk using a cake ring. Place the ring around the sponge. Spread the crunch on top of the sponge and freeze until you prepare Nutella mousse.
Prepare Nutella Mousse. Place chocolate and ½ cup whipping cream into a heatproof bowl and melt over bain-marie. Add nutella and stir to combine.
Meanwhile, soak gelatin powder in water for 5-10 minutes. Dissolve bloomed gelatin over low heat. Pour over the chocolate mixture.
In a large bowl mix chilled cream until soft peaks form. Add chocolate mixture and mix to combine. Transfer the mousse into a piping bag fitted with a plain tip.
Assemble the cake. Place the frozen sponge crunch into the center of a serving platter. Place a 8 inch (20cm) ring on top of it and line it with an acetate sheet for easier removal.
Pipe the mousse on sides and top. Smooth the top using an offset spatula.
Cover and refrigerate for 4-6 hour or overnight.
Prepare chocolate glaze. Place chocolate and cream in heatproof bowl and place over a pan with simmering water. Melt over low heat.
Use a spoon and start making drips around the edges. If the ganache hardens too much, heat it a bit over bain-marie to get to a pouring consistency. When you finish all the drips on the edges, fill the top with ganache and spread it evenly using the offset spatula. Refrigerate for at least 30 minutes before decorating.
Decorate the cake with Ferrero Rocher truffles, chocolate shards and chopped hazelnuts.