01 - Set a heatproof bowl over a saucepan of barely simmering water to create a bain-marie. Add the dark chocolate and unsalted butter, stirring gently until completely smooth and glossy. Remove from heat and allow to cool for 5 minutes.
02 - In a mixing bowl, beat the egg yolks with half the granulated sugar until the mixture turns pale yellow, thickens, and falls in ribbons from the whisk.
03 - Pour the slightly cooled melted chocolate into the egg 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 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 additions, using a large spatula with slow, sweeping motions to preserve maximum air volume.
06 - Whip the chilled heavy cream in a cold bowl until soft peaks form. Delicately fold the whipped cream into the mousse until just incorporated and the texture is uniform.
07 - Spoon or pipe the mousse into individual serving glasses or ramekins. Cover tightly with plastic wrap and refrigerate for at least 2 hours to set.
08 - Serve chilled, garnished with dark chocolate shavings or a small dollop of whipped cream if desired. Pairs beautifully with ruby port or a shot of espresso.