01 - Set a heatproof bowl over a saucepan of barely simmering water. Add the dark chocolate and butter, stirring gently until completely melted and smooth. Remove from heat and allow to cool for 5 minutes.
02 - Place the egg yolks in a mixing bowl with half of the sugar. Whisk vigorously until the mixture turns pale yellow, thickens slightly, and falls in ribbons from the whisk.
03 - Pour the melted chocolate into the yolk mixture and stir until evenly blended and glossy.
04 - In a clean, dry bowl, beat the egg whites with a pinch of salt until soft peaks begin to form. Slowly add the remaining sugar while continuing to beat until stiff, glossy peaks hold their shape.
05 - Fold the whipped egg whites into the chocolate mixture in three additions, using a large spatula and gentle sweeping motions to preserve as much volume as possible.
06 - Whip the chilled heavy cream to soft peaks. Fold it into the mousse base with light strokes until just combined and no white streaks remain.
07 - Transfer the mousse into individual serving glasses or ramekins. Cover tightly with plastic wrap and refrigerate for at least 2 hours until firm and set.
08 - Serve chilled, garnished with chocolate shavings, a dollop of whipped cream, or a dusting of cocoa powder as desired.