Melt the dark chocolate in a heatproof bowl over a pot of simmering water (double boiler method). Stir until smooth, then let it cool slightly.

In a separate bowl, whip the heavy cream until soft peaks form. Set aside.

In another bowl, beat the egg whites until frothy. Gradually add the sugar and continue beating until stiff peaks form.

Gently fold the melted chocolate into the whipped cream until combined.

Carefully fold in the beaten egg whites in two batches, being cautious not to deflate the mixture.

Spoon the mousse into individual serving glasses or bowls.

Chill in the refrigerator for at least 2 hours or until set.