01 - Melt dark chocolate in a heatproof bowl set over simmering water, stirring until smooth. Remove from heat and allow to cool slightly.
02 - Whisk egg yolks with half the sugar until pale and creamy. Stir in the melted chocolate and vanilla extract.
03 - In a separate bowl, beat egg whites with a pinch of salt until soft peaks form. Gradually add remaining sugar and continue beating until stiff peaks develop.
04 - Whip heavy cream until soft peaks form.
05 - Gently fold whipped cream into the chocolate mixture until just combined. Carefully fold in beaten egg whites in two additions, preserving the airiness.
06 - Divide mousse evenly into 4 serving vessels. Cover and refrigerate for at least 2 hours to set.
07 - Whip remaining heavy cream with powdered sugar and vanilla extract until soft peaks form.
08 - Top chilled mousse with the whipped cream. Garnish optionally with chocolate shavings, cocoa powder, or fresh berries.