01 - Melt the chopped dark chocolate and butter together in a heatproof bowl set over a pot of simmering water (double boiler) until smooth. Remove from heat and let cool slightly.
02 - In a separate bowl, beat the egg yolks with half the sugar (1.5 tablespoons) until pale and creamy.
03 - Stir the melted chocolate mixture into the egg yolk mixture until well combined.
04 - In a clean bowl, whip the heavy cream until soft peaks form.
05 - In another clean bowl, beat the egg whites until foamy, then 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 in the beaten egg whites, being careful not to deflate the mixture.
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 if desired.