Melt the chocolate: Break the dark chocolate into small pieces and melt it in a heatproof bowl over a pot of simmering water (double boiler). Stir occasionally until smooth. Remove from heat and let cool slightly.

Whip the egg whites: In a clean, dry bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add half the sugar and continue beating until stiff peaks form.

Prepare the egg yolks: In another bowl, whisk the egg yolks with the remaining sugar until pale and creamy. Add the vanilla extract and mix well.

Combine chocolate and yolks: Gently fold the melted chocolate into the egg yolk mixture until fully combined.

Fold in whipped cream: Whip the heavy cream until soft peaks form, then gently fold it into the chocolate mixture.

Incorporate egg whites: Carefully fold the whipped egg whites into the chocolate mixture in two batches, ensuring not to deflate the mixture.

Chill the mousse: Divide the mousse into serving glasses or bowls. Refrigerate for at least 4 hours, or overnight, to set.

Serve: Garnish with whipped cream, chocolate shavings, or fresh berries before serving.