01 - Melt chopped 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 of the granulated sugar until pale and thick, then stir in vanilla extract.
03 - Gradually incorporate the melted chocolate into the egg yolk mixture, stirring constantly until fully blended.
04 - In a clean bowl, whip egg whites with a pinch of salt until soft peaks form. Gradually add remaining sugar and beat until stiff, glossy peaks appear.
05 - Whip the 0.5 cup of heavy cream until soft peaks form.
06 - Gently fold the whipped cream into the chocolate mixture, then carefully fold in the beaten egg whites in three additions to preserve volume.
07 - Spoon the mixture into serving glasses or bowls, cover, and refrigerate for a minimum of 2 hours until set.
08 - Beat remaining heavy cream with powdered sugar and vanilla extract until soft peaks form.
09 - Top each set mousse portion with a generous dollop of freshly whipped cream just before serving.