01 - Set a heatproof bowl over a saucepan of barely 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, whisk the egg yolks with half of the sugar vigorously until the mixture turns pale yellow and falls in thick ribbons from the whisk, about 2 minutes.
03 - Pour the slightly cooled melted chocolate into the yolk mixture, folding with a spatula until uniformly blended and no streaks remain.
04 - In a clean, dry bowl, beat the egg whites with a pinch of salt using an electric mixer on medium speed until foamy. Increase to high speed and gradually add the remaining sugar, beating until stiff, glossy peaks hold their shape.
05 - Add the whipped egg whites to the chocolate mixture in three additions, folding gently with a spatula using broad strokes. Work carefully to preserve as much air as possible — the batter should remain light and voluminous.
06 - In a chilled bowl, whip the heavy cream to soft peaks — it should hold its shape but still droop slightly when the whisk is lifted. Fold the whipped cream into the mousse base with a light hand until just combined and no white streaks remain.
07 - Transfer the mousse into individual serving glasses or ramekins, dividing evenly. Cover tightly with plastic wrap and refrigerate for a minimum of 2 hours, or until the mousse is firmly set and chilled through.
08 - Garnish each portion with chocolate shavings or a dollop of freshly whipped cream if desired. Serve immediately while still cold.