01 - Set a heatproof bowl over a saucepan of barely simmering water to create a bain-marie. Add the dark chocolate and butter, stirring gently until completely smooth and glossy. Remove from heat and allow to cool for 3 to 5 minutes.
02 - Place the egg yolks in a mixing bowl with half the sugar. Whisk vigorously until the mixture turns pale yellow, doubles in volume, and falls in thick ribbons from the whisk.
03 - Pour the slightly cooled melted chocolate into the egg yolk mixture, folding with a spatula until uniformly blended 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 foamy. Increase to high speed, gradually adding the remaining sugar, and continue beating until stiff, glossy peaks hold their shape.
05 - Add the whipped egg whites to the chocolate mixture in three additions, folding gently with a spatula using slow, sweeping motions. Work from the bottom upward to preserve as much air as possible.
06 - Whip the chilled heavy cream in a separate bowl until soft peaks form. Fold the whipped cream into the mousse base in two additions, handling delicately until just combined and no white streaks remain.
07 - Divide the mousse among four serving glasses or ramekins. Cover each tightly with plastic wrap and refrigerate for at least 2 hours, or until the mousse is firmly set.
08 - Remove from the refrigerator just before serving. Garnish with chocolate shavings, a dollop of whipped cream, or a dusting of cocoa powder as desired.