01 - Combine chopped dark chocolate and unsalted butter in a heatproof bowl over simmering water and melt until smooth. Remove from heat and let cool slightly.
02 - In a separate bowl, beat the egg yolks with half the sugar until pale and creamy.
03 - Stir the melted chocolate mixture into the egg yolk mixture until fully incorporated.
04 - Whip the heavy cream in a clean bowl until soft peaks form.
05 - In another clean bowl, beat the egg whites until foamy, gradually add the remaining sugar, and continue beating until stiff peaks form.
06 - Gently fold the whipped cream into the chocolate mixture until just combined.
07 - Carefully fold the beaten egg whites into the chocolate mixture, preserving the airiness.
08 - Spoon the mousse into serving glasses or bowls, cover, and refrigerate for at least 1 hour until set.
09 - Before serving, garnish with whipped cream, grated chocolate, and fresh berries as desired.