01 - In a heatproof bowl set over a pan of simmering water (double boiler), melt the chocolate with the butter, stirring until smooth. Remove from heat and let cool slightly.
02 - In a large bowl, whisk the egg yolks with half the sugar (2 tbsp) and vanilla extract until pale and creamy. Slowly whisk in the cooled chocolate mixture until well combined.
03 - In a clean, grease-free bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and continue beating until stiff, glossy peaks form.
04 - In a separate bowl, whip the cold heavy cream until soft peaks form.
05 - Gently fold the whipped cream into the chocolate mixture until just combined.
06 - Carefully fold in the beaten egg whites in three additions, using a spatula, until the mixture is smooth and airy—do not overmix.
07 - Spoon or pipe the mousse into individual serving glasses or ramekins. Refrigerate for at least 2 hours, or until set.
08 - Before serving, garnish with fresh berries, chocolate shavings, and mint leaves if desired.