01 - Place chopped dark chocolate in a heatproof bowl set over a pot of simmering water. Stir continuously until completely smooth and melted. Remove from heat source and allow to cool for 5 minutes.
02 - In a thoroughly clean mixing bowl, beat egg whites until soft peaks form. Gradually sprinkle in granulated sugar while continuing to beat until stiff, glossy peaks develop.
03 - Lightly whisk egg yolks with vanilla extract in a separate bowl. Pour melted chocolate into yolk mixture, stirring constantly until fully incorporated and smooth.
04 - In a chilled bowl, whip heavy cream until soft peaks form. Be careful not to over-whip.
05 - Gently fold whipped cream into chocolate-yolk mixture using a spatula. Carefully incorporate beaten egg whites in two separate additions, maintaining airy texture throughout.
06 - Divide mousse evenly among individual serving glasses or ramekins. Cover with plastic wrap and refrigerate for minimum 2 hours or until completely set.
07 - Combine strawberries, raspberries, and blueberries in mixing bowl. Lightly toss with powdered sugar if additional sweetness is desired.
08 - Remove chilled mousse from refrigerator. Top each portion generously with mixed berry mixture. Garnish with fresh mint leaves and chocolate shavings if using. Serve immediately while cold.