Begin by melting the dark chocolate chips in a heatproof bowl over a pot of simmering water (double boiler method). Stir until smooth, then remove from heat and let cool slightly.
In another bowl, whip the heavy cream until soft peaks form. Be careful not to overwhip. Gently fold the whipped cream into the cooled melted chocolate until well combined.
In a separate bowl, whisk together the hazelnut spread and vanilla extract until smooth. Gently fold this mixture into the chocolate and cream mixture.
In a clean bowl, whip the egg whites and a pinch of salt until soft peaks form. Gradually add the sugar, continuing to whip until stiff peaks form.
Gently fold the egg whites into the chocolate mixture, being careful not to deflate the mousse. Fold until just incorporated.
Spoon the mousse into small dessert cups or glasses, filling them about three-quarters full.
Cover and refrigerate for at least 2 hours to set.
Before serving, garnish with crushed hazelnuts and a dollop of whipped cream on top, if desired.
Notes
For a richer flavor, use high-quality dark chocolate.