Melt the dark chocolate and unsalted butter together in a heatproof bowl set over a pot of simmering water, stirring gently until blended.

Let the mixture cool slightly.

In a separate bowl, beat the egg yolks with the granulated sugar until pale and creamy.

Stir in the chocolate mixture along with the vanilla extract.

In another bowl, whip the heavy cream until soft peaks form.

Gently fold the whipped cream into the chocolate mixture.

In a clean bowl, beat the egg whites with a pinch of salt until stiff peaks form, then carefully fold them into the mousse.

Spoon the mousse into serving dishes and refrigerate for about an hour until set.