01 - Set a heatproof bowl over a saucepan of gently simmering water (bain-marie), ensuring the bowl does not touch the water. Add the chopped dark chocolate and cubed butter, stirring occasionally until completely smooth and glossy. Remove from heat and let cool for 5 minutes until lukewarm.
02 - In a mixing bowl, vigorously whisk the egg yolks with half of the granulated sugar (about 0.9 oz) until the mixture turns pale yellow, thickens slightly, and falls in ribbons from the whisk.
03 - Pour the melted chocolate mixture into the whisked egg yolks, folding gently with a spatula until uniformly blended and no streaks of yolk remain.
04 - In a separate clean, dry bowl, beat the egg whites with a pinch of salt using an electric mixer on medium speed until soft peaks form. Gradually sprinkle in the remaining sugar while continuing to beat on high speed until stiff, glossy peaks hold their shape.
05 - Using a large spatula, gently fold the whipped egg whites into the chocolate mixture in three additions. Cut through the center and sweep along the edge of the bowl, rotating as you fold, to preserve maximum airiness.
06 - In a chilled bowl, whip the cold heavy cream to soft peaks — it should hold its shape when the whisk is lifted but still appear smooth. Fold the whipped cream into the mousse base in two additions until just combined and no white streaks remain.
07 - Divide the mousse among four serving glasses or ramekins using a spoon or piping bag. Smooth the tops lightly, cover each with plastic wrap, and refrigerate for at least 2 hours or until firmly set.
08 - Remove the mousses from the refrigerator 10 minutes before serving. Garnish with dark chocolate shavings, a dollop of lightly whipped cream, or fresh berries as desired.