Melt the chocolate: Break the dark chocolate into small pieces and melt it in a heatproof bowl over a pot of simmering water (double boiler). Stir occasionally until smooth. Remove from heat and let it cool slightly.

Prepare the egg yolks: In a separate bowl, whisk the egg yolks with half of the sugar until pale and creamy. Add the vanilla extract and mix well.

Combine chocolate and yolks: Gradually add the melted chocolate to the egg yolk mixture, stirring continuously until fully combined.

Whip the egg whites: In a clean, dry bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and continue beating until stiff peaks form.

Fold in the egg whites: Gently fold the whipped egg whites into the chocolate mixture in three additions, being careful not to deflate the mixture.

Chill the mousse: Divide the mousse into serving glasses or bowls. Cover and refrigerate for at least 4 hours, or overnight, to set.

Serve: Before serving, optionally garnish with a dollop of whipped cream and fresh berries.