01 - Set a heatproof bowl over a pot of simmering water to create a bain-marie. Add the chopped dark chocolate and stir continuously until completely smooth and melted. Remove from heat source and allow to cool for a few minutes.
02 - In a thoroughly clean mixing bowl, beat the egg whites using an electric mixer or whisk until soft peaks begin to form. Gradually sprinkle in the granulated sugar while continuing to beat until stiff, glossy peaks develop that hold their shape.
03 - In a separate bowl, whisk the egg yolks together with the vanilla extract until lightly blended. Pour the cooled melted chocolate into the yolk mixture, stirring constantly until completely smooth and fully incorporated.
04 - In a chilled bowl, whip the cold heavy cream using a mixer or whisk until soft peaks form—the cream should hold its shape but still have a slightly floppy texture.
05 - Gently fold the whipped cream into the chocolate-yolk mixture using a spatula, being careful to maintain as much air as possible. Then fold in the beaten egg whites in two separate additions, mixing until just fully incorporated and the mixture appears light and airy.
06 - Divide the mousse evenly among individual serving glasses or ramekins. Cover each container with plastic wrap and refrigerate for a minimum of 2 hours, or until the mousse is completely set and firm.
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 the berries evenly.
08 - Remove the chilled mousse from the refrigerator. Top each portion with a generous layer of the mixed berry mixture. Garnish with fresh mint leaves and chocolate shavings if desired. Serve immediately while cold.