Melt chocolate and butter together in a heatproof bowl over simmering water or in the microwave until smooth.
Let the melted chocolate cool slightly.
Whisk egg yolks into the cooled chocolate mixture until fully combined.
In a separate bowl, beat egg whites with salt until soft peaks form, then gradually add sugar and beat until stiff peaks form.
In another bowl, whip heavy cream and vanilla extract until soft peaks form.
Gently fold whipped cream into the chocolate mixture.
Carefully fold in beaten egg whites in batches until no streaks remain.
Spoon mousse into serving glasses or bowls.
Refrigerate for at least 1 hour before serving.
