Step by step recipe:

1. 3 min.
   Preheat the oven to 320°F (160°C).
   
   Take 200 g liquid cream out of the fridge.
   
   Lay a sheet of cooking parchment on a baking sheet and spread the 100 g ground hazelnuts out on this.
2. 20 min.
   Toast in the oven for 20 minutes. This dry roasting brings out the full flavour of the hazelnuts.
   
   Leave to cool.
3. 7 min.
   Heat a bain-marie and melt 280 g chocolate.
4. 2 min.
   When the chocolate is completely melted, turn off the heat and add 200 g liquid cream (not too cold).
   
   Mix well.
5. 4 min.
   Fold in 100 g egg yolk, then the toasted ground hazelnuts.
6. 5 min.
   Beat 200 g egg white with 1 pinch fine (or table) salt.
7. 2 min.
   Important: the egg white should not be beaten as stiff as for meringue, but only to the "soft" stage, where it just about sticks to the whisk.
   
   This is the secret of a successful chocolate mousse.
   
   Finish by folding in 100 g caster sugar.
8. 3 min.
   Start by pouring a little of the chocolate mixture into the beaten egg whites and mix in gently (we want to keep the mousse light and fluffy).
9. 3 min.
   Then pour this mixture into the rest of the egg whites and fold in gently.
10. 
    To make transferring the mousse into dishes easier, you can use a forcing bag.
11. 6 min.
    Stand the dishes on a cloth laid on a baking sheet. This makes it easier to move them quickly without them sliding about.
    
    Fill the dishes and cover with plastic film. Refrigerate for at least 1 hour.
12. 
    Now just sit back and enjoy watching your guests marvel at the light texture and sublime flavour of this chocolate-hazelnut mousse!