01 - Melt the chocolate in a heatproof bowl over a pot of simmering water, stirring until smooth. Remove from heat and let cool slightly.
02 - In a medium bowl, whisk the egg yolks and 2 tbsp of the sugar until pale and thick. Stir in the vanilla extract.
03 - Gradually add the melted chocolate to the yolk mixture, stirring constantly until fully combined.
04 - In a clean bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining 2 tbsp sugar and beat until stiff, glossy peaks form.
05 - In another bowl, whip the 0.5 cup 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, taking care not to deflate the mixture.
07 - Spoon the mousse into serving glasses or bowls. Cover and refrigerate for at least 2 hours, or until set.
08 - For the whipped cream, beat the 0.5 cup heavy cream with powdered sugar and vanilla extract until soft peaks form.
09 - Just before serving, top each mousse with a generous dollop of whipped cream.