01 - Melt chopped dark chocolate in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and let cool slightly.
02 - Whisk egg yolks with half of the granulated sugar until pale and creamy, then fold in melted chocolate and vanilla extract.
03 - In a clean bowl, beat egg whites with a pinch of salt until soft peaks form. Gradually add remaining sugar and continue beating until stiff peaks appear.
04 - Whip cold heavy cream until soft peaks form.
05 - Gently fold whipped cream into the chocolate and egg yolk mixture until combined. Then carefully fold in beaten egg whites in two additions, maintaining airiness.
06 - Divide mousse evenly into 4 serving vessels, cover, and refrigerate for at least 2 hours until set.
07 - Whip remaining heavy cream with powdered sugar and vanilla extract until soft peaks form.
08 - Top chilled mousse with prepared whipped cream. Optionally garnish with chocolate shavings, cocoa powder, or berries.