01 - Set a heatproof bowl over a saucepan of barely simmering water, ensuring the bowl does not touch the water. Add the chopped dark chocolate and cubed 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 along with half of the granulated sugar. Beat vigorously with an electric mixer or whisk for 2–3 minutes until the mixture turns pale yellow, thickens, and falls in ribbons from the whisk.
03 - Pour the lukewarm melted chocolate mixture into the beaten egg yolks. Fold and stir with a spatula until fully incorporated and no streaks remain.
04 - In a separate clean, dry bowl, beat the egg whites with a pinch of salt using clean beaters until soft peaks begin to form. Gradually stream in the remaining sugar while continuing to beat until stiff, glossy peaks hold their shape when the whisk is lifted.
05 - Gently fold the whipped egg whites into the chocolate mixture in three additions. Use a large spatula with slow, sweeping motions from the bottom to the top of the bowl, rotating as you go, to preserve as much air as possible.
06 - In a chilled bowl, whip the cold heavy cream until soft peaks form. Delicately fold the whipped cream into the mousse base until just combined and uniform in color. Avoid overmixing to maintain the airy texture.
07 - Divide the mousse evenly among four serving glasses or ramekins. Cover each tightly with plastic wrap and refrigerate for a minimum of 2 hours, or until the mousse is fully set and holds its shape.
08 - Remove from the refrigerator just before serving. Garnish with dark chocolate shavings or a small dollop of freshly whipped cream if desired. Serve chilled.