01 - Set a heatproof bowl over a saucepan of barely simmering water, ensuring the bowl does not touch the water. Add the chopped dark chocolate and cubed butter, stirring gently until completely smooth and glossy. Remove from heat and allow to cool for 5 minutes.
02 - In a medium mixing bowl, vigorously whisk the egg yolks with half of the granulated sugar until the mixture turns pale yellow, doubles in volume, and falls in thick ribbons from the whisk.
03 - Pour the slightly cooled melted chocolate into the egg yolk mixture, folding with a spatula until fully incorporated and no streaks remain. The mixture should be smooth and homogeneous.
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 begin to form. Gradually sprinkle in the remaining sugar while continuing to beat until stiff, glossy peaks hold their shape when the beaters are lifted.
05 - Add the whipped egg whites to the chocolate mixture in three equal additions. Fold gently using a large spatula with slow, sweeping motions from the bottom to the top, rotating the bowl a quarter turn after each fold. Maintain as much air in the batter as possible.
06 - In a chilled bowl, whip the cold heavy cream to soft peaks — it should hold its shape but still appear silky and billowy. Fold the whipped cream into the mousse base in two additions using the same gentle folding technique until just combined and no white streaks remain.
07 - Divide the mousse evenly among four serving glasses or ramekins, smoothing the tops with a spatula. Cover each tightly with plastic wrap and refrigerate for at least 2 hours, or until the mousse is firm and set throughout.
08 - Remove from the refrigerator 5 minutes before serving. Garnish with dark chocolate shavings or a small dollop of freshly whipped cream if desired. Serve immediately.