01 - Place the chopped dark chocolate in a heatproof bowl set over a pan of gently simmering water, ensuring the bowl does not touch the water. Stir continuously until completely smooth and melted, then remove from heat. Allow to cool to lukewarm before proceeding.
02 - In a clean, dry bowl, beat the egg whites with a pinch of salt using an electric mixer or whisk until soft peaks form. Gradually add half of the granulated sugar while continuing to beat until stiff, glossy peaks form. Set aside.
03 - In a separate bowl, whisk together the egg yolks, remaining granulated sugar, and vanilla extract until the mixture becomes pale yellow and creamy in texture, about 2-3 minutes.
04 - Gradually fold the cooled melted chocolate into the egg yolk mixture, using a rubber spatula with gentle folding motions until fully incorporated and no streaks remain.
05 - In a chilled bowl, whip the cold heavy cream until soft peaks form, taking care not to over-whip as the mixture should still be slightly pliable.
06 - Gently fold the whipped cream into the chocolate-yolk mixture using light, lifting motions until just combined. Some streaks are acceptable at this stage.
07 - Add one-third of the beaten egg whites to the chocolate mixture and fold gently to lighten. Repeat with the remaining egg whites in two more additions, folding carefully to maintain as much air as possible without deflating the mixture.
08 - Divide the mousse evenly among serving glasses or bowls. Cover tightly with plastic wrap and refrigerate for a minimum of 2 hours, or until completely set and firm to the touch.
09 - Just before serving, place the halved strawberries in a bowl. Toss gently with powdered sugar and lemon juice if using, allowing the berries to macerate slightly for enhanced sweetness.
10 - Remove the chilled mousse from the refrigerator. Top each serving with the prepared strawberries, arranging them attractively. Garnish with chocolate shavings and fresh mint leaves if desired. Serve immediately.