01 - Set a heatproof bowl over a saucepan of barely simmering water, ensuring the bottom does not touch the water. Add the chopped dark chocolate and butter, stirring gently until completely melted and smooth. Remove from heat and allow to cool for about 5 minutes until lukewarm.
02 - Place the egg yolks in a mixing bowl with half of the granulated sugar. Whisk vigorously for 2 to 3 minutes until the mixture turns pale yellow and falls in thick ribbons from the whisk.
03 - Pour the cooled melted chocolate into the yolk mixture in a steady stream, folding gently with a spatula until evenly incorporated and no streaks remain.
04 - In a separate clean, dry bowl, beat the egg whites with a pinch of salt using an electric mixer on medium speed until soft peaks begin to form. Slowly add the remaining sugar while continuing to beat, increasing to high speed, until stiff and glossy peaks hold their shape.
05 - Add the whipped egg whites to the chocolate mixture in three separate additions. Fold each batch in gently with a large spatula, using slow sweeping motions from bottom to top, being careful not to deflate the air incorporated into the whites.
06 - Pour the chilled heavy cream into a cold mixing bowl and whip with an electric mixer on medium-high speed until soft peaks form. Fold the whipped cream into the mousse base in two additions, stirring minimally to maintain an airy, light texture throughout.
07 - Transfer the finished mousse into individual serving glasses or ramekins using a spoon or piping bag. Smooth the tops, cover each tightly with plastic wrap, and refrigerate for a minimum of 2 hours to allow the mousse to set properly.
08 - Remove from the refrigerator just before serving. Garnish with dark chocolate shavings or a small dollop of freshly whipped cream if desired. Pairs excellently alongside a glass of ruby port or a freshly pulled espresso.