Bring the heavy cream to the boil in a medium saucepan. Off the heat, add the chocolate pieces and stir with a wooden spoon or spatula, working from the middle and stirring in the same direction. Gradually incorporate the chocolate on the edges, and keep stirring until all the chocolate is melted.
In a stand mixer, or using a hand mixer, whip the egg whites, while gradually incorporating the sugar, to medium peaks (not too stiff: when you bring the whip up from the whites, you should have a bec d’oiseau or “bird’s beak”).
Add the egg yolks to the cream-chocolate mix and stir well. Transfer this mixture to a large bowl, then delicately and gently fold in the egg whites just until the whites have disappeared.
Turn the mixture into a large serving bowl, or into 6-8 individual ramekins, and refrigerate for at least two hours.