For the mousse: Fill a small saucepot halfway with water and bring to a boil. Put chocolate, butter and salt into a heat-resistant medium bowl and place the bowl on top of the pot. When the chocolate has melted, about 5 minutes, stir it with a spatula to blend. Remove from the heat and let cool slightly. Whisk the egg yolks and vanilla into the cooled chocolate; the mixture will thicken as it cools.
In a medium bowl, beat the egg whites with an electric mixer on medium speed until foamy, 1 to 2 minutes. Add 2 tablespoons of the sugar and continue beating until stiff peaks form.
In a separate bowl, beat the cream with an electric mixer until frothy. Add the remaining 2 tablespoons sugar and continue beating until stiff peaks form.
Using a spatula, fold the whipped egg whites into the chocolate mixture in batches and gently blend until no streaks remain. Next, fold in the whipped cream in batches until blended.
Spoon a heaping 1/4 cup of the mousse into six 4-ounce ramekins or serving cups. Cover and chill for at least 2 hours.
For the whipped cream: Using an electric mixer, whip the cream until frothy. Add the sugar and liqueur and beat until stiff peaks form.
To serve, spoon a dollop of whipped cream onto the mousse and sprinkle with toasted hazelnuts and shaved chocolate.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.