Melt the chocolate: In a heatproof bowl, combine the chopped chocolate and butter. Set over a pot of simmering water, stirring occasionally until melted and smooth. Remove from heat and let cool slightly.

Whisk the egg yolks: In another bowl, whisk together the egg yolks, sugar, and vanilla extract until thick and creamy, about 2 minutes.

Combine mixtures: Gradually add the melted chocolate to the egg yolk mixture, stirring continuously until well combined.

Beat the egg whites: In a clean bowl, using clean beaters, whisk the egg whites with a pinch of salt until stiff peaks form.

Fold in the egg whites: Gently fold the beaten egg whites into the chocolate mixture, being careful not to deflate the fluffy whites.

Whip the cream: In a separate bowl, whip the chilled heavy cream until soft peaks form.

Combine everything: Gently fold the whipped cream into the chocolate mixture until no streaks remain.

Chill: Spoon the mousse into individual serving dishes or one large bowl and refrigerate for at least 2 hours, ideally overnight.