01 - Place chopped dark chocolate in a heatproof bowl over simmering water and stir until smooth. Remove from heat and allow to cool slightly.
02 - Whisk egg yolks with half the granulated sugar until pale and creamy. Incorporate melted chocolate and vanilla extract.
03 - In a separate bowl, whisk egg whites with a pinch of salt until soft peaks form. Gradually add remaining sugar and continue to whisk until stiff peaks develop.
04 - Whip the heavy cream until soft peaks form.
05 - Gently fold the whipped cream into the chocolate mixture until incorporated. Carefully fold in the beaten egg whites in two batches, maintaining volume.
06 - Divide mousse evenly among four serving glasses or bowls. Cover and refrigerate for at least two hours until set.
07 - Whip remaining heavy cream with powdered sugar and vanilla extract until soft peaks form.
08 - Top chilled mousse with freshly whipped cream. Optionally garnish with chocolate shavings, cocoa powder, or berries.