Melt the dark chocolate in a double boiler.
Whisk egg yolks with sugar until thick and pale. Add to melted chocolate while stirring constantly.
Whip heavy cream into stiff peaks, then fold gently into the chocolate mixture.
In another bowl, beat egg whites until foamy, gradually adding more sugar as you continue beating them till they form soft peaks. Gently fold in about one-third of the whipped cream to lighten it before incorporating all at once and completely.