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, 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. 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 overbeat, or the cream will become grainy.
06 - Gently fold the whipped cream into the chocolate-egg yolk base using a spatula, working in smooth, sweeping motions to preserve the airiness.
07 - Fold the beaten egg whites into the mousse in three separate additions. Use a light hand and cut downward through the center, sweeping along the bottom and up the sides to maintain maximum volume.
08 - Spoon or pipe the finished mousse into individual serving glasses or ramekins. Refrigerate for at least 2 hours, or until the mousse is fully set and holds its shape when served.