01 - Combine 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 5 minutes.
02 - Place the egg yolks in a mixing bowl with half of the granulated sugar. Beat vigorously using an electric mixer until the mixture turns pale yellow, thickens, and falls in ribbons from the whisk.
03 - Pour the slightly cooled melted chocolate into the yolk mixture in a steady stream, folding gently with a spatula until uniformly blended and no streaks remain.
04 - In a spotlessly clean bowl, beat the egg whites with a pinch of salt until soft peaks begin to form. Sprinkle in the remaining sugar gradually while continuing to beat until stiff, glossy peaks hold their shape.
05 - Transfer the whipped egg whites into the chocolate base in three separate additions. Fold each batch slowly and deliberately using a large spatula, cutting through the center and sweeping along the edges to preserve maximum airiness.
06 - Pour the chilled heavy cream into a cold bowl and whip until soft peaks just form. Gently fold the whipped cream into the mousse with minimal strokes until evenly distributed and the texture is uniform.
07 - Divide the mousse among individual serving glasses or ramekins. Cover tightly with plastic wrap and refrigerate for a minimum of 2 hours, or until the mousse is firmly set and chilled throughout.
08 - Remove from the refrigerator just before serving. Top with chocolate shavings, a dollop of whipped cream, or a light dusting of cocoa powder as desired.