Prepare a Double Boiler:

• Fill a saucepan with 1 inch of water and bring it to a simmer.

• Place the chopped chocolate and butter in a heatproof bowl and set it over the saucepan, ensuring the bottom of the bowl doesn’t touch the water.

Melt the Chocolate and Butter:

• Stir the chocolate and butter until smooth and fully melted.

• Remove from heat and stir in the hazelnut liqueur and vanilla extract.

• Let the mixture cool slightly while preparing the eggs.

Whisk the Egg Yolks and Sugar:

• In a separate bowl, whisk together egg yolks and half of the sugar (2 tbsp) until thick, pale, and creamy (about 2-3 minutes).

• Gradually add the slightly cooled chocolate mixture, whisking to combine.

• Set aside.

Beat the Egg Whites:

• In a clean mixing bowl, beat the egg whites and salt until soft peaks form.

Add Sugar and Whip to Stiff Peaks:

• Gradually add the remaining 2 tbsp sugar and continue beating until stiff peaks form and the mixture is glossy.

• Be careful not to overbeat.

Beat the Heavy Cream:

• In another chilled bowl, beat the heavy cream until soft peaks form.

• Be careful not to over-whip.

Combine the Mixture:

• Gently fold the whipped cream into the chocolate mixture until just combined.

Incorporate the Egg Whites:

• Gently fold the egg whites into the chocolate mixture in two additions, using a spatula and being careful not to deflate the mixture.

• The mousse should be light, airy, and fully incorporated.

Divide the Mousse:

• Spoon the mousse evenly into 6 serving glasses or ramekins.

• Smooth the tops with a spatula.

Chill the Mousse:

• Refrigerate for at least 2 hours or until set and firm.

Garnish the Mousse:

• Just before serving, top each mousse with a dollop of whipped cream, a sprinkle of crushed hazelnuts, and chocolate shavings.

• Add a mint leaf for a pop of color (optional).

Serve and Enjoy:

• Serve chilled for the best texture and flavor.