Finely chop the Dark Chocolate and place it in a heatproof bowl with the Unsalted Butter. Set the bowl over barely simmering water (or gently microwave in short bursts), stirring until fully melted and smooth. Remove from heat and let cool slightly.
Separate the Eggs, placing the yolks in one bowl and the whites in another; keep everything cool for best volume.
Whisk the egg yolks with the Granulated Sugar until the mixture is pale and slightly thickened. Fold this yolk-sugar mixture into the warm chocolate mixture until homogeneous and shiny.
Add a very small pinch of Salt to the egg whites and beat them to soft peaks, then continue beating until you have glossy, medium-stiff peaks.
Gently fold one-third of the whipped egg whites into the chocolate mixture to loosen it, then carefully fold in the remaining whites in two additions, using a light hand to preserve the airiness.
Spoon the mousse into individual serving dishes or a single bowl, cover, and chill in the refrigerator for at least 2 hours (or until set) to develop texture and flavor.
Serve chilled as-is or with optional garnishes such as whipped cream, fresh berries, a dusting of cocoa powder, or shaved chocolate.
