01 - Place the dark chocolate and unsalted butter in a heatproof bowl set over a saucepan of gently simmering water. Stir continuously until completely smooth and glossy. Remove from heat and allow to cool for 3 to 5 minutes.
02 - In a mixing bowl, whisk the egg yolks with half of the granulated sugar vigorously until the mixture turns pale yellow and falls in thick ribbons from the whisk.
03 - Pour the slightly cooled melted chocolate into the egg yolk mixture and stir until uniformly blended with no streaks remaining.
04 - In a clean, grease-free bowl, beat the egg whites with a pinch of salt using an electric mixer until soft peaks begin to form. Slowly add the remaining sugar while continuing to beat until stiff, glossy peaks hold their shape.
05 - Using a large spatula, gently fold the whipped egg whites into the chocolate mixture in three separate additions. Use a light sweeping motion from the bottom to the top to preserve maximum air volume.
06 - Pour the chilled heavy cream into a cold bowl and whip until soft peaks form. Gently fold the whipped cream into the mousse base with a spatula until just combined and the mixture is uniformly silky.
07 - Transfer the mousse into individual serving glasses or ramekins using a spoon or piping bag. Cover each portion tightly with plastic wrap and refrigerate for at least 2 hours to set properly.
08 - Remove from the refrigerator and serve chilled. Garnish with dark chocolate shavings, a dollop of whipped cream, or fresh berries as desired.