In a microwave-safe bowl, combine the chopped dark chocolate and butter. Microwave in short bursts, stirring in between until fully melted and smooth. Set aside to cool.

In a mixing bowl, whisk the egg yolks and granulated sugar until the mixture is light and creamy. Stir in the vanilla extract.

Gradually fold the melted chocolate mixture into the egg yolk mixture until well blended.

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

In a clean bowl, whisk the egg whites with a pinch of salt until soft peaks form. Carefully fold the egg whites into the chocolate mixture, alternating with the whipped cream, until just combined.

Transfer the mousse mixture into a prepared cake pan and refrigerate for at least 1 hour, or until set.

Once chilled, carefully remove the cake from the pan. Decorate with your favorite toppings, slice, and enjoy!