Melt the dark chocolate in a heatproof bowl over simmering water until smooth. Let cool slightly.

Whip the heavy cream until soft peaks form; set aside.

In another bowl, whisk egg yolks with sugar until pale and creamy.

Fold melted chocolate into the egg yolk mixture, then gently fold in half of the whipped cream. Finally, fold in the remaining whipped cream.

In a clean bowl, whip egg whites until stiff peaks form, then carefully fold into the chocolate mixture.

Spoon into serving dishes and chill for at least 2 hours or overnight before serving.