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.