01 - Set up a double boiler by placing a heatproof bowl over a pot of simmering water. Add the chopped dark chocolate and stir continuously until completely smooth and melted. Remove from heat and allow to cool for 5 minutes.
02 - In a clean mixing bowl, whisk together the egg yolks, half of the granulated sugar (1.5 tablespoons), and vanilla extract. Beat vigorously until the mixture becomes pale yellow and creamy in texture.
03 - Gently fold the cooled melted chocolate into the egg yolk mixture using a spatula. Incorporate slowly until fully combined and no streaks remain, being careful not to overmix.
04 - In a chilled bowl, whip the cold heavy cream using an electric mixer or whisk until soft peaks form. The cream should hold its shape briefly when the whisk is lifted. Set aside.
05 - Using a completely clean bowl and beaters, beat the egg whites with a pinch of salt until soft peaks appear. Gradually sprinkle in the remaining sugar while continuing to beat until stiff, glossy peaks form.
06 - First, gently fold the whipped cream into the chocolate mixture until just incorporated. Then add the beaten egg whites in two separate additions, folding with a light hand to maintain the airy structure of the mousse.
07 - Distribute the mousse evenly among serving glasses or bowls. Cover with plastic wrap and refrigerate for a minimum of 2 hours, or until completely set and firm to the touch.
08 - Just before serving, place the strawberries, blueberries, and raspberries in a bowl. Toss gently with powdered sugar if using, coating the berries lightly for a touch of sweetness.
09 - Remove the chilled mousse from the refrigerator. Top each portion generously with the mixed berry mixture and garnish with fresh mint leaves if desired. Serve immediately.