01 - Gently melt the chopped dark chocolate in a heatproof bowl set over simmering water, stirring until smooth. Remove from heat and allow to cool slightly.
02 - Whisk the egg yolks with half the granulated sugar until pale and creamy, then fold in the melted chocolate and vanilla extract.
03 - In a clean bowl, whisk the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and continue beating until stiff peaks develop.
04 - Whip the heavy cream until soft peaks form.
05 - Gently fold the whipped cream into the chocolate yolk mixture until just combined. Then carefully fold in the beaten egg whites in two additions to preserve volume.
06 - Distribute the mousse evenly into four serving glasses. Cover and refrigerate for a minimum of two hours until set.
07 - Whip the remaining heavy cream with powdered sugar and vanilla extract until soft peaks form.
08 - Top the chilled mousse with the prepared whipped cream. Optionally garnish with chocolate shavings, cocoa powder, or fresh berries.