Begin by melting the dark chocolate in a heatproof bowl over a saucepan of simmering water (double boiler method). Stir occasionally until smooth, then remove from heat and let cool slightly.
In a separate bowl, beat the egg yolks with the sugar until the mixture is pale and creamy. Gradually mix in the melted chocolate and vanilla extract until well combined.
In another mixing bowl, whip the heavy cream until it forms soft peaks. Gently fold the whipped cream into the chocolate mixture, being careful not to deflate it.
In a clean bowl, beat the egg whites with a pinch of salt until stiff peaks form. Gently fold the beaten egg whites into the chocolate mixture until no white streaks remain.
Pour the mousse into serving cups or bowls and refrigerate for at least 2 hours, or until set. Serve chilled, garnished with berries or whipped cream if desired.
