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 cubed butter, stirring gently until completely melted and smooth. 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 for about 2 minutes 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 in a steady stream, folding gently with a spatula until uniformly blended and glossy.
04 - In a spotlessly clean bowl, beat the egg whites with a pinch of salt using an electric mixer on medium speed until soft peaks begin to form. Increase to high speed and gradually sprinkle in the remaining sugar, beating until stiff, glossy peaks hold their shape when the beaters are lifted.
05 - Scoop one-third of the beaten egg whites into the chocolate batter and stir vigorously to lighten the base. Gently fold in the remaining egg whites in two additions using a large spatula, cutting through the center and folding along the edge to preserve maximum airiness.
06 - Pour the chilled heavy cream into a cold bowl and whip to soft peaks. Delicately fold the whipped cream into the mousse in two additions, using broad sweeping motions until just combined and no white streaks remain.
07 - Transfer the mousse into four serving glasses or ramekins using a spoon or piping bag. Smooth the tops, cover tightly with plastic wrap, and refrigerate for a minimum of 2 hours until firmly set.
08 - Remove from the refrigerator 10 minutes before serving. Garnish with dark chocolate shavings, a dollop of softly whipped cream, or fresh berries as desired.