01 - Place the chopped dark chocolate in a heatproof bowl set over a pot of simmering water, creating a bain-marie. Stir continuously until the chocolate melts completely and achieves a smooth consistency. Remove from heat and allow to cool slightly.
02 - In a clean, dry mixing bowl, beat the egg whites using an electric mixer or whisk until soft peaks form. Gradually sprinkle in the granulated sugar while continuing to beat. Continue mixing until stiff, glossy peaks form that hold their shape.
03 - In a separate bowl, lightly whisk the egg yolks with the pure vanilla extract. Pour the slightly cooled melted chocolate into the yolk mixture, stirring constantly until fully incorporated and the mixture becomes smooth and uniform.
04 - Using a chilled bowl and beaters, whip the cold heavy cream until soft peaks form. Be careful not to over-whip, as the cream should still have some movement and not become stiff.
05 - Gently fold one-third of the whipped cream into the chocolate-yolk mixture to lighten it. Add the remaining whipped cream and fold until just incorporated. Carefully fold in the beaten egg whites in two separate additions, using a spatula with a gentle lifting motion to maintain the airy texture.
06 - Divide the mousse evenly among individual serving glasses, ramekins, or decorative bowls. Cover each container with plastic wrap or place on a tray and cover. Refrigerate for at least 2 hours, or until the mousse is completely set and firm to the touch.
07 - Just before serving, combine the strawberries, raspberries, and blueberries in a mixing bowl. If desired, sprinkle with powdered sugar and toss gently to coat evenly. Adjust sweetness according to berry ripeness.
08 - Remove the chilled mousse from the refrigerator. Top each serving generously with the prepared mixed berry mixture. Garnish with fresh mint leaves and chocolate shavings if using. Serve immediately while cold for the best texture and flavor experience.