01 - Melt the dark chocolate in a heatproof bowl set over a pot of gently simmering water (double boiler). Stir until smooth, then remove from heat and let cool slightly.
02 - In a separate bowl, whip the cold heavy cream with vanilla extract until soft peaks form. Refrigerate until needed.
03 - In another bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add granulated sugar and continue beating until stiff, glossy peaks form.
04 - Stir the egg yolks into the cooled, melted chocolate until well combined.
05 - Gently fold the whipped cream into the chocolate mixture using a spatula.
06 - Carefully fold in the beaten egg whites in two additions, ensuring the mixture stays light and airy.
07 - Spoon the mousse evenly into 4 serving glasses or ramekins. Cover and refrigerate for at least 2 hours, or until set.
08 - Before serving, toss the halved strawberries with powdered sugar (if using) and arrange them atop the mousse. Garnish with fresh mint leaves if desired.