01 - Set a heatproof bowl over a saucepan of gently simmering water (bain-marie). 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 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 — be careful not to overwhip, as the cream should remain supple and flow slightly.
06 - Gently fold the whipped cream into the chocolate-egg yolk base using a spatula, working in smooth, sweeping motions from the bottom upward to preserve airiness.
07 - Carefully fold the beaten egg whites into the mousse in three separate additions, using a light hand to avoid deflating the batter. The mixture should remain voluminous and silky.
08 - Divide the mousse evenly among individual serving glasses or ramekins. Refrigerate uncovered for at least 2 hours, or until the mousse is fully set and firms to a spoonable consistency.