Begin by melting the dark chocolate. Place the chopped chocolate in a heatproof bowl over a pot of simmering water, ensuring the bowl does not touch the water. Stir occasionally until the chocolate is completely melted and smooth. Remove from heat and let it cool slightly.

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

In another bowl, beat the heavy cream with the vanilla extract until soft peaks form. Be careful not to overwhip.

Once the melted chocolate has cooled slightly, stir in the egg yolks until well combined.

Gently fold in one-third of the whipped cream into the chocolate mixture to lighten it. Then, carefully fold in the remaining whipped cream until no white streaks remain.

Next, gently fold in the whipped egg whites, taking care to maintain the light and airy texture.

Divide the mousse into serving dishes or glasses. Cover and refrigerate for at least 2 hours or until set.

Serve chilled, optionally garnished with chocolate shavings or a dollop of whipped cream.