01 - Place chocolate and butter in a heatproof bowl over simmering water and stir until smooth, then remove from heat and let cool slightly.
02 - In a clean bowl, beat egg yolks with half the sugar (2 tablespoons) until pale and creamy, then stir in vanilla extract and melted chocolate mixture until fully combined.
03 - In a separate clean bowl, whisk egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar (2 tablespoons) and continue whisking until stiff peaks form.
04 - Whip the heavy cream in another bowl until soft peaks form.
05 - Gently fold the whipped cream into the chocolate mixture until just combined.
06 - Carefully fold the egg whites into the mixture in two additions, using a spatula and taking care not to deflate the mousse.
07 - Spoon the mousse evenly into six small serving cups or ramekins.
08 - Cover and refrigerate for at least 2 hours until set.
09 - Before serving, optionally garnish with whipped cream, chocolate shavings, or fresh berries.