01 - Set a heatproof bowl over a saucepan of barely simmering water. Add the chopped dark chocolate and cubed butter, stirring gently until completely smooth and glossy. Remove from heat and allow to cool for 5 minutes.
02 - In a mixing bowl, whisk the egg yolks with half the granulated sugar until the mixture turns pale yellow and falls in thick ribbons from the whisk.
03 - Pour the slightly cooled melted chocolate into the yolk mixture, folding with a spatula until fully incorporated and no streaks remain.
04 - In a 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 sprinkle in the remaining sugar while continuing to beat until stiff, glossy peaks hold their shape.
05 - Gently fold the whipped egg whites into the chocolate mixture in three separate additions, using a large spatula with slow, sweeping motions to preserve as much air as possible.
06 - In a chilled bowl, whip the heavy cream to soft peaks. Fold it into the mousse base in two additions, stopping as soon as the mixture is uniform and silky.
07 - Divide the mousse among serving glasses or ramekins. Cover each with plastic wrap and refrigerate for at least 2 hours, or until firmly set.
08 - Serve chilled, topped with chocolate shavings, a dollop of whipped cream, or a light dusting of cocoa powder as desired.