01 - Melt the dark chocolate in a heatproof bowl set over a pot of simmering water, stirring continuously until smooth. Remove from heat and allow to cool for 5 minutes.
02 - In a clean, dry bowl, beat egg whites until soft peaks form. Gradually add sugar and continue beating until stiff, glossy peaks form, approximately 3-4 minutes.
03 - Lightly beat egg yolks with vanilla extract in a separate bowl. Stir in the cooled melted chocolate until thoroughly incorporated and smooth.
04 - Whip chilled heavy cream in a cold bowl until soft peaks form, being careful not to overwhip.
05 - Gently fold whipped cream into chocolate mixture until partially combined. Add beaten egg whites in two additions, folding carefully until fully incorporated and no white streaks remain.
06 - Distribute mousse evenly into individual serving glasses or ramekins. Cover with plastic wrap and refrigerate for minimum 2 hours or until fully set.
07 - Combine strawberries, raspberries, and blueberries in a bowl. Toss with powdered sugar if additional sweetness is desired.
08 - Top each chilled mousse with mixed berries. Garnish with fresh mint leaves and chocolate shavings if desired. Serve immediately while cold.