Begin by preparing a double boiler. Fill the bottom pot with water and bring it to a simmer. Place the dark chocolate in the top pot or a heatproof bowl over the simmering water. Stir until melted and smooth. Remove from heat and let it cool slightly.

In a mixing bowl, whisk the egg whites with a pinch of salt until soft peaks form. Gradually add in the granulated sugar, continuing to whisk until glossy and stiff peaks form.

In another bowl, using a whisk or electric mixer, whip the heavy cream with the vanilla extract until you achieve soft peaks.

Gently combine the egg yolks into the melted chocolate, mixing well to fully incorporate.

First, fold one-third of the beaten egg whites into the chocolate mixture to lighten it. Then gradually fold in the remaining egg whites until no white streaks remain. Be gentle to keep the mixture airy.

Finally, fold in the whipped cream gently until fully incorporated and smooth.

Divide the mousse into serving cups or ramekins and refrigerate for at least 2 hours or until set.

Serve chilled, optionally topped with whipped cream or shaved chocolate.