01 - Place dark chocolate in a heatproof bowl set over a pot of simmering water, ensuring the bowl doesn't touch the water. Stir continuously until completely smooth and melted. Remove from heat and allow to cool for 5 minutes until lukewarm.
02 - In a clean mixing bowl, whisk egg yolks with half the granulated sugar (1.5 tablespoons) and vanilla extract. Beat vigorously until the mixture becomes pale yellow and slightly thickened, approximately 2-3 minutes.
03 - Pour the cooled melted chocolate into the egg yolk mixture in a steady stream. Gently fold using a spatula with sweeping motions until fully incorporated and no streaks remain.
04 - In a chilled bowl, beat cold heavy cream with an electric mixer or whisk until soft peaks form when the beaters are lifted. The cream should hold its shape briefly but still appear glossy. Set aside.
05 - Using a completely clean bowl and beaters, beat egg whites with a pinch of salt on medium speed until foamy. Gradually sprinkle in remaining sugar while continuing to beat. Increase speed to high and whip until stiff, glossy peaks form that don't droop when the beaters are lifted.
06 - Gently fold one-third of the whipped cream into the chocolate base to lighten it. Add remaining cream and fold until just combined. Carefully fold in the beaten egg whites in two additions, using minimal strokes to maintain airiness and prevent deflation.
07 - Divide the mousse evenly among 4 serving glasses or bowls. Cover each with plastic wrap touching the surface to prevent a skin from forming. Refrigerate for at least 2 hours or until completely set and firm to the touch.
08 - Just before serving, place strawberries, blueberries, and raspberries in a mixing bowl. Sprinkle with powdered sugar if using and toss gently to coat evenly.
09 - Remove mousse from refrigerator. Top each serving with an even portion of the sugared berries. Garnish with fresh mint leaves if desired. Serve immediately while berries are fresh.