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 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 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. Be careful not to overwhip.
06 - Gently fold the whipped cream into the chocolate-egg yolk base using a spatula, using slow sweeping motions to preserve as much air as possible.
07 - Carefully fold the beaten egg whites into the mousse in three separate additions. Use a light hand to avoid deflating the mixture, ensuring an airy, uniform consistency.
08 - Spoon or pipe the finished mousse into individual serving glasses or ramekins. Refrigerate for at least 2 hours, or until fully set and firm to the touch.