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 slightly while preparing other components.
02 - In a medium bowl, whisk egg yolks with half the granulated sugar and vanilla extract until the mixture becomes pale yellow and creamy in texture. Slowly pour in the melted chocolate mixture while whisking constantly to prevent cooking the eggs, combining until fully incorporated and smooth.
03 - Using a clean bowl and electric mixer or whisk, beat the cold heavy cream until soft peaks form when the whisk is lifted. The cream should hold its shape but still appear glossy. Set aside in the refrigerator until needed.
04 - In a completely clean, grease-free bowl, beat egg whites with a pinch of salt starting at low speed. Gradually increase speed and add the remaining sugar one tablespoon at a time. Continue beating until glossy, stiff peaks form and the whites hold their shape firmly when the whisk is inverted.
05 - Gently fold one-third of the whipped cream into the chocolate mixture using a spatula, incorporating with light folding motions to maintain airiness. Add the remaining whipped cream and fold until just combined. Carefully fold in the beaten egg whites in two separate additions, being gentle to preserve the mousse's light texture.
06 - Portion the mousse evenly into four elegant serving glasses, smoothing the tops with a spatula. Cover each glass with plastic wrap and refrigerate for a minimum of 2 hours, or until completely set and firm to the touch.
07 - Just before serving, combine fresh berries with powdered sugar and lemon juice in a small bowl, tossing gently to coat evenly. Spoon the berry mixture over each chilled mousse. Garnish with fresh mint leaves and chocolate shavings if desired. Serve immediately while berries are fresh.