Start by melting the dark chocolate in a heatproof bowl over a pot of simmering water (double boiler method) or in the microwave in short intervals, stirring frequently until smooth. Remove from heat and let cool slightly.
In a separate bowl, whisk the egg yolks with the sugar until they are pale and creamy. Stir in the melted chocolate and hazelnut spread until well combined.
In another clean bowl, whip the heavy cream with the vanilla extract until soft peaks form. Gently fold the whipped cream into the chocolate mixture until just combined.
In a clean bowl, beat the egg whites with a pinch of salt until stiff peaks form. Carefully fold the beaten egg whites into the chocolate mixture in three additions to maintain the airy texture.
Once fully combined, spoon the mousse into small cups or ramekins. Refrigerate for at least 2 hours to set.
Before serving, garnish each mousse cup with crushed hazelnuts and fresh mint leaves if desired.
Notes
Allow the mousse to set in the refrigerator for at least 2 hours for the best texture.