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 clean, grease-free 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 yolk-sugar 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. Be careful not to over-whip, as the cream should remain smooth and spreadable.
06 - Gently fold the whipped cream into the chocolate-egg yolk base using a spatula. Use broad, sweeping strokes to maintain as much air as possible in the mixture.
07 - Fold the beaten egg whites into the mousse in three separate additions. For each addition, use a light hand and gentle folding motion to preserve the airy texture. The mixture should be uniform but still light and fluffy.
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 the mousse is fully set and chilled through.