Fill a 2-quart / 1.9 L saucepan with about 2 inches of water and bring to a bare simmer over low heat.

Set a heatproof bowl over the saucepan without letting it touch the water. Add the chopped dark chocolate and 120 ml of the heavy cream, stirring until fully melted and smooth.

Remove the bowl from the heat and let the chocolate mixture cool to room temperature, about 15 minutes. It should feel neutral to the touch, not warm.

In a separate large bowl, whip the remaining 240 ml heavy cream with the powdered sugar, vanilla extract, and salt using a hand mixer, until soft peaks form, about 2 minutes. Stop before the cream looks stiff or grainy.

Add one-third of the whipped cream to the cooled chocolate mixture and fold gently with a rubber spatula to lighten it.

Fold in the remaining whipped cream in two more additions, using wide strokes, until no white streaks remain and the mousse looks airy.

Divide the mousse between 6 serving glasses or ramekins, about 120 ml each.

Cover and chill for at least 2 hours, until the mousse holds a clear spoon mark and doesn't slump when tilted.

Garnish with chocolate shavings or fresh berries just before serving.