In a bowl, whip the heavy cream with a hand mixer until soft peaks form.

Separate the eggs, placing the egg whites in a clean mixing bowl and the yolks in the whipped cream bowl.

Fold the egg yolks into the whipped cream until the mixture is combined with no yolk streaks left behind.

Melt the semisweet chocolate and unsalted butter together in a heatproof bowl over a pot of simmering water, stirring until smooth. Or heat in the microwave at 30 second intervals, stirring between.

Gently fold the whipped cream into the chocolate mixture in two additions until just combined. Set aside.

Using a clean hand mixer, beat the egg whites until foamy, then gradually add the white sugar, continuing to beat until stiff peaks form.

Carefully fold in the beaten egg whites into the chocolate mixture, ensuring the mousse remains light and airy.

Divide the mousse evenly among serving dishes and refrigerate for at least 6 hours or until set.

Serve chilled, optionally garnished with whipped cream, chocolate shavings, or fresh berries.