- In a heatproof bowl, combine the chopped chocolate and butter. Set the bowl over a pot of simmering water (double boiler method) and stir until melted and smooth. Remove from heat and let cool slightly. 
- In a separate bowl, beat the egg yolks with the granulated sugar and vanilla extract until the mixture is pale and thick. Gradually stir in the melted chocolate mixture until well combined. 
- In another bowl, whip the heavy cream until soft peaks form. Gently fold the whipped cream into the chocolate mixture until no streaks remain. 
- In a clean bowl, beat the egg whites with a pinch of salt until stiff peaks form. Carefully fold the egg whites into the chocolate mixture in three additions, being gentle to maintain the airy texture. 
- Spoon the mousse into individual serving dishes or a large bowl. Cover and refrigerate for at least 2 hours, or until set. 
- Serve chilled, garnished with whipped cream, chocolate shavings, or fresh berries if desired.