01 - Combine dark chocolate and butter in a heatproof bowl set over simmering water. Stir continuously until smooth, then remove from heat and allow to cool slightly.
02 - In a clean, dry bowl, whisk egg whites with salt until soft peaks form. Gradually incorporate half the sugar and continue whisking until firm, glossy peaks develop.
03 - In a separate bowl, whisk egg yolks with remaining sugar and vanilla extract until pale and slightly thickened.
04 - Fold the melted chocolate mixture into the egg yolk mixture until fully blended.
05 - Gently fold one-third of the whipped egg whites into the chocolate-yolk blend to lighten it, then carefully fold in the remaining whites in two additions until no streaks remain.
06 - Transfer mousse into serving glasses or ramekins. Cover and refrigerate for a minimum of 2 hours until set.
07 - Prior to serving, adorn with whipped cream, chocolate shavings, or fresh berries as desired.