01 - Set a heatproof bowl over a saucepan of gently simmering water. Add the chopped dark chocolate and butter, stirring occasionally until completely smooth. Remove from heat and let cool slightly for about 5 minutes.
02 - In a spotlessly clean bowl, beat the egg whites with a pinch of salt using an electric mixer until soft peaks begin to form. Gradually sprinkle in half of the sugar while continuing to beat until stiff, glossy peaks hold their shape.
03 - In a separate bowl, whisk the egg yolks with the remaining sugar vigorously until the mixture turns pale yellow and falls in thick ribbons from the whisk.
04 - Pour the slightly cooled chocolate mixture into the beaten egg yolks. Fold gently with a spatula until fully incorporated and no streaks remain.
05 - In a chilled bowl, whip the cold heavy cream until soft peaks form. Be careful not to overwhip, as the cream should remain silky and spreadable.
06 - Gently fold the whipped cream into the chocolate-egg yolk base using broad, sweeping motions with a spatula to preserve the airy texture.
07 - Carefully fold the stiff egg whites into the mixture in three separate additions, using a light hand to avoid deflating the batter. The final mixture should be voluminous and cloud-like.
08 - Spoon or pipe the mousse into individual serving glasses or ramekins. Refrigerate uncovered for at least 2 hours, or until firmly set. Cover with plastic wrap if chilling longer than 3 hours.