01 - Set a heatproof bowl over a saucepan of simmering water to create a bain-marie. Add the dark chocolate and butter, stirring gently until completely smooth and glossy. Remove from heat and allow to cool for 5 minutes.
02 - In a mixing bowl, vigorously whisk the egg yolks with half of the granulated sugar until the mixture turns pale yellow and falls in thick ribbons from the whisk.
03 - Pour the melted chocolate mixture into the egg yolk mixture, folding steadily until fully incorporated and no streaks remain.
04 - In a separate clean bowl, beat the egg whites with a pinch of salt using an electric mixer until soft peaks begin to form. Gradually sprinkle in the remaining sugar while continuing to beat until stiff, glossy peaks hold their shape.
05 - Gently fold the whipped egg whites into the chocolate base in three separate additions, using a large spatula and slow sweeping motions to preserve as much air as possible.
06 - Whip the chilled heavy cream to soft peaks. Delicately fold it into the mousse in two additions until the mixture is uniform, silky, and light.
07 - Transfer the mousse into individual serving glasses or ramekins. Cover tightly with plastic wrap and refrigerate for a minimum of 2 hours until fully set and firm.
08 - Remove from the refrigerator and garnish with chocolate shavings or a small dollop of whipped cream if desired. Serve immediately.