Place the chopped chocolate in a heatproof bowl. Melt it over a pot of simmering water (double boiler method), stirring occasionally until smooth. Alternatively, you can melt it in the microwave in 30-second bursts, stirring in between. Once melted, let it cool slightly.
In a separate bowl, whisk together the egg yolks and half of the sugar (2 tablespoons) until the mixture is pale and slightly thickened.
Gently fold the melted chocolate into the egg yolk mixture. Add the vanilla extract and mix until well combined.
In another clean, dry bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar while continuing to beat until stiff peaks form.
Carefully fold the egg whites into the chocolate mixture in two batches, being careful not to deflate the whites. Fold just until no white streaks remain.
In a separate bowl, whip the heavy cream until it forms soft peaks. Gently fold the whipped cream into the chocolate mixture until well incorporated, ensuring the mixture remains light and airy.
Spoon the mousse into individual serving glasses or a large serving dish. Chill in the refrigerator for at least 2 hours, or until set.
Before serving, garnish the chocolate mousse with a dollop of whipped cream, chocolate shavings, or fresh berries, if desired.