01 - Set a heatproof bowl over a saucepan of barely simmering water, ensuring the bottom does not touch the water. Add the chopped dark chocolate and butter, stirring gently until completely melted 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. Whisk vigorously using an electric mixer or balloon whisk until the mixture turns pale yellow, thickens, and falls in ribbons from the whisk, approximately 2 to 3 minutes.
03 - Pour the slightly cooled melted chocolate into the yolk mixture in a steady stream, folding and stirring until the two are fully incorporated and the mixture is uniform in color with no streaks.
04 - In a spotlessly clean bowl, beat the egg whites with a pinch of salt using clean beaters on medium speed until soft peaks begin to form. Gradually sprinkle in the remaining sugar while continuing to beat, increasing speed slightly, until stiff, glossy peaks hold their shape when the whisk is lifted.
05 - Add the whipped egg whites to the chocolate mixture in three additions. For each addition, use a large spatula to fold gently from the bottom of the bowl upward and over the top, rotating the bowl as you go, until no white streaks remain. Work deliberately but lightly to preserve the air volume.
06 - Pour the chilled heavy cream into a cold mixing bowl and whip to soft peaks. Delicately fold the whipped cream into the mousse base in two additions using the same folding motion, stopping as soon as the mixture appears smooth, uniform, and airy.
07 - Divide the mousse among 4 serving glasses or ramekins using a spoon or piping bag. Smooth the tops lightly, cover each with plastic wrap, and refrigerate for a minimum of 2 hours, or until the mousse is firmly set and chilled through.
08 - Remove the mousses from the refrigerator just before serving. Garnish with dark chocolate shavings, a small dollop of whipped cream, or a light dusting of cocoa powder as desired. Serve chilled.