01 - Melt the chopped dark chocolate in a heatproof bowl placed over gently simmering water. Stir until smooth, then remove from heat and allow to cool slightly.
02 - Whisk the egg yolks with half the sugar and vanilla extract until pale and creamy.
03 - Gradually blend the melted chocolate into the egg yolk mixture until fully combined.
04 - In a clean bowl, whisk egg whites with a pinch of salt until soft peaks form. Slowly add the remaining sugar and continue whipping to stiff, glossy peaks.
05 - Whip the heavy cream in a separate bowl until soft peaks develop.
06 - Gently fold the whipped cream into the chocolate and egg yolk mixture.
07 - Carefully fold the beaten egg whites into the mixture in two additions, preserving the mousse's light texture.
08 - Spoon or pipe the mousse into serving cups, cover, and refrigerate for at least 2 hours until set.
09 - Prior to serving, top each cup with whipped cream, chocolate shavings, and fresh berries if desired.