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 it 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: Slowly pour the melted chocolate into the egg yolk mixture, stirring continuously until fully combined.

Fold in egg whites: Gently fold the whipped egg whites into the chocolate mixture in two batches, being careful not to deflate the mixture.

Whip the cream: In a separate bowl, whip the heavy cream until it forms soft peaks. Fold the whipped cream into the chocolate mixture until fully incorporated.

Chill the mousse: Divide the mousse into serving glasses or bowls. Cover with plastic wrap and refrigerate for at least 4 hours, or overnight, to set.

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