01 - Melt the dark chocolate in a heatproof bowl set over a pot of simmering water (bain-marie), stirring until smooth. Remove from heat and let cool slightly.
02 - In a clean bowl, beat the egg whites until soft peaks form. Gradually add the sugar and continue beating until stiff, glossy peaks form.
03 - In another bowl, lightly beat the egg yolks with the vanilla extract. Stir in the melted chocolate until smooth and well combined.
04 - In a separate bowl, whip the heavy cream until soft peaks form.
05 - Gently fold the whipped cream into the chocolate mixture, then carefully fold in the beaten egg whites in two additions, until fully incorporated and airy.
06 - Spoon the mousse into individual serving glasses or bowls. Cover and refrigerate for at least 2 hours, or until set.
07 - Before serving, combine the berries in a bowl and toss with powdered sugar if desired.
08 - Top each mousse with the mixed berries, garnish with fresh mint leaves and chocolate shavings if using. Serve chilled.