01 - Set a heatproof bowl over a saucepan of barely simmering water, ensuring the bowl does not touch the water. Add the chopped dark chocolate and butter, stirring gently until completely melted and smooth. Remove from heat and allow to cool for about 5 minutes.
02 - In a mixing bowl, whisk the egg yolks with half of the sugar vigorously until the mixture turns pale yellow and falls in thick ribbons from the whisk, about 2 minutes.
03 - Pour the slightly cooled melted chocolate into the egg yolk mixture and fold together until uniformly blended and no streaks remain.
04 - In a separate clean, dry bowl, beat the egg whites with a pinch of salt using an electric mixer on medium speed until soft peaks begin to form. Gradually add the remaining sugar while continuing to beat, increasing to high speed, until stiff, glossy peaks hold their shape.
05 - Gently fold the whipped egg whites into the chocolate mixture in three additions, using a spatula with slow, sweeping motions to preserve as much air as possible. The batter should remain light and voluminous.
06 - In a chilled bowl, whip the heavy cream to soft peaks. Fold the whipped cream into the mousse base with gentle strokes until just combined and no white streaks remain.
07 - Transfer the mousse into individual serving glasses or ramekins, smoothing the tops. Cover tightly with plastic wrap and refrigerate for at least 2 hours, or until the mousse is firmly set.
08 - Serve chilled, garnished with dark chocolate shavings, a dollop of whipped cream, or a light dusting of cocoa powder as desired.