01 - Melt the dark chocolate in a heatproof bowl over a pot of simmering water (double boiler), stirring until smooth. Remove from heat and let cool slightly.
02 - In a clean bowl, whisk the egg yolks with half of the sugar and vanilla extract until pale and creamy.
03 - Gently fold the melted chocolate into the egg yolk mixture until well combined.
04 - In a separate bowl, whip the heavy cream until soft peaks form. Set aside.
05 - In another clean bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and continue to beat until stiff peaks form.
06 - Carefully fold the whipped cream into the chocolate mixture, then gently fold in the beaten egg whites in two additions, taking care not to deflate the mousse.
07 - Spoon the mousse into serving glasses or bowls. Cover and refrigerate for at least 2 hours, or until set.
08 - Just before serving, toss the mixed berries with powdered sugar if desired.
09 - Top each mousse with mixed berries and garnish with fresh mint leaves.