01 - Gently melt the chopped dark chocolate and butter in a heatproof bowl placed over simmering water until smooth. Remove from heat and allow to cool slightly.
02 - In a separate bowl, whisk the egg yolks with half of the sugar until pale and creamy.
03 - Stir the melted chocolate mixture into the egg yolk mixture until fully integrated.
04 - In a clean bowl, whip the heavy cream until soft peaks form.
05 - In another clean bowl, beat the egg whites until foamy, gradually adding the remaining sugar and continuing until stiff peaks develop.
06 - Gently fold the whipped cream into the chocolate mixture until just combined.
07 - Carefully fold the beaten egg whites into the mixture to maintain its airy texture.
08 - Spoon the mousse into serving glasses or bowls, cover, and refrigerate for at least one hour until set.
09 - Before serving, optionally garnish with whipped cream, grated chocolate, and fresh berries.