01 - Combine chopped dark chocolate and unsalted butter in a heatproof bowl placed over simmering water and melt until smooth. Remove from heat and allow to cool slightly.
02 - In a separate bowl, beat the egg yolks with half of the sugar (1.5 tablespoons) until pale and creamy.
03 - Stir the slightly cooled chocolate mixture into the egg yolk blend until fully incorporated.
04 - Whip the heavy cream in a clean bowl until soft peaks form.
05 - In another clean bowl, beat egg whites until foamy, then gradually add the remaining sugar (1.5 tablespoons) and continue beating until stiff peaks form.
06 - Gently fold the whipped cream into the chocolate and egg yolk mixture until just combined.
07 - Carefully fold the beaten egg whites into the mousse base, preserving the airiness of the mixture.
08 - Transfer the mixture into serving glasses or bowls, cover, and refrigerate for at least 1 hour until set.
09 - Before serving, top with whipped cream, grated chocolate or cocoa powder, and fresh berries as desired.