01 - Set up a double boiler by placing a heatproof bowl over a saucepan of gently simmering water. Add the chopped dark chocolate and butter, stirring occasionally until completely smooth and glossy. Remove from heat and allow to cool for 5 minutes.
02 - Place the egg yolks in a mixing bowl with half of the sugar. Beat vigorously with an electric mixer on medium-high speed until the mixture turns pale yellow, thickens, and falls in ribbons from the beaters, approximately 3 minutes.
03 - Pour the slightly cooled melted chocolate into the whipped yolk mixture, folding gently with a spatula until completely incorporated and no streaks remain.
04 - In a clean, dry bowl, beat the egg whites with a pinch of salt using clean beaters until soft peaks begin to form. Slowly sprinkle in the remaining sugar while continuing to beat until the whites hold stiff, glossy peaks that stand upright when the whisk is lifted.
05 - Gently fold the beaten 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. The final mixture should be light and uniform in color.
06 - Pour the chilled heavy cream into a cold bowl and whip until soft peaks form. Delicately fold the whipped cream into the mousse base in two additions until just combined and silky smooth. Avoid overmixing.
07 - Divide the mousse evenly among four serving glasses or ramekins. Cover each tightly with plastic wrap and refrigerate for at least 2 hours, or until firmly set and chilled through.
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. Serve chilled.