01 - Place the chopped dark chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water (bain-marie). Stir occasionally until completely smooth, then remove from heat and let cool 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 add half 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 egg yolk mixture and fold gently with a spatula until fully incorporated and no streaks remain.
05 - In a chilled bowl, whip the cold heavy cream with an electric mixer until soft peaks form — do not overwhip, as the cream should remain supple and smooth.
06 - Gently fold the whipped cream into the chocolate-egg yolk base using broad, sweeping strokes with a spatula to maintain as much air as possible.
07 - Carefully fold the beaten egg whites into the mousse in three separate additions, using a light hand to avoid deflating the mixture. The final result should be airy and uniform in color.
08 - Spoon or pipe the finished mousse into individual serving glasses or ramekins. Cover loosely with plastic wrap and refrigerate for at least 2 hours, or until firmly set.