01 - Place chocolate and butter in a heatproof bowl set over simmering water, creating a double boiler. Stir continuously until completely smooth and melted. Remove from heat and allow to cool for 5 minutes.
02 - In a medium bowl, whisk egg yolks with 2 tablespoons of sugar and vanilla extract until the mixture becomes pale yellow and creamy, approximately 2 minutes. Gradually pour in the cooled chocolate mixture while whisking constantly until fully incorporated and smooth.
03 - In a chilled bowl, beat 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 in the refrigerator.
04 - In a separate clean, dry bowl, beat egg whites with a pinch of salt until soft peaks appear. Gradually add the remaining sugar while continuing to beat until stiff, glossy peaks form and the mixture stands upright when the beaters are lifted.
05 - Gently fold one-third of the whipped cream into the chocolate base to lighten the mixture. Add the remaining whipped cream and fold until just incorporated. Carefully fold in the beaten egg whites in two additions, using a spatula with a gentle lifting motion to maintain the airy texture. Stop folding as soon as no white streaks remain visible.
06 - Divide the mousse evenly among four serving glasses or ramekins. Cover with plastic wrap and refrigerate for a minimum of 2 hours, or until the mousse is completely set and firm to the touch.
07 - In a small bowl, gently toss the fresh berries with powdered sugar and lemon juice until evenly coated. Allow to sit for 5 minutes to release natural juices.
08 - Remove the chilled mousse from the refrigerator. Spoon the prepared berry mixture over the top of each mousse portion. Garnish with fresh mint leaves and chocolate shavings if desired. Serve immediately.