Melt chocolate: Begin by placing the dark chocolate in a heatproof bowl over simmering water. Stir gently until smooth, then let it cool slightly.

Whip cream: In a separate bowl, whip the heavy cream until soft peaks form. Reserve a dollop for topping.

Whisk yolks and sugar: Separate the eggs, placing the yolks in a bowl. Whisk with granulated sugar until pale and thick.

Fold in chocolate: Gently fold the melted chocolate into the yolk mixture, then fold in the whipped cream.

Beat egg whites: In a clean bowl, beat the egg whites until stiff peaks form, then fold into the chocolate mixture.

Chill mousse: Spoon the mousse into serving cups and refrigerate for at least 2 hours.