01 - Combine chopped dark chocolate and unsalted butter in a heatproof bowl over simmering water (double boiler) until melted and smooth; remove from heat and let cool slightly.
02 - Whisk egg yolks with half of the sugar (1.5 tbsp) until pale and creamy.
03 - Stir the melted chocolate mixture into the egg yolk mixture until fully incorporated.
04 - Whip heavy cream in a clean bowl until soft peaks form.
05 - In another clean bowl, beat egg whites until foamy; gradually add remaining sugar (1.5 tbsp) and continue beating until stiff peaks develop.
06 - Gently fold whipped cream into the chocolate mixture until just combined.
07 - Carefully fold in the beaten egg whites, taking care not to deflate the mixture.
08 - Spoon mousse into individual serving glasses or bowls, cover, and refrigerate for at least 1 hour to set.
09 - Optionally, top with whipped cream, grated chocolate, and fresh berries before serving.