Bring 320ml double cream to the boil in a saucepan. Break the chocolate into small pieces if necessary and place in a heatproof bowl.
When the cream comes to the boil, pour the cream onto the chocolate and stir until all of the chocolate has melted and you have a glossy ganache. Allow the ganache to cool slightly for around 5 minutes.
In another bowl, lightly whisk the remaining 430ml double cream to soft peaks.
Fold this cream into the ganache until evenly distributed.
Transfer the mixture to a piping bag and pipe the mixture into 6 glasses. Refrigerate for at least 30 minutes to allow the mousse to set.
Whilst the chocolate mix is cooling, make the cream: Scrape the seeds from the vanilla pod. Sieve the icing sugar into the double cream and add the vanilla. Transfer to a syphon and charge with 1 canister of gas. Shake well and refrigerate until ready to use.
Preheat oven to 180 degrees.
Roast the hazelnuts for 6-7 minutes or until starting to turn golden brown.
Make a caramel in a heavy based pan. When ready tip onto a tray lined with a silicon mat and sprinkle the hazelnuts straight on top so they stick to the caramel.
Allow to cool for 5 minutes.
Blitz in a machine for 30 seconds so the nuts are completely mixed into the caramel dust.
Remove the glasses from the fridge 10 minutes before serving, squirt the cream on top followed by a generous sprinkle of hazelnut crumble
«Try replacing the dark chocolate with milk chocolate and replacing the hazelnuts with almonds for a much lighter mousse.»