Set up a heatproof bowl over a saucepan with a little simmering water (double boiler setup) and add the chopped chocolate and butter, stirring until melted and smooth.

Remove the heatproof bowl from the saucepan and allow the chocolate mixture to cool slightly.

In a clean mixing bowl, use an electric mixer or whisk to beat the egg whites with salt until soft peaks form. Gradually add sugar, continuing to beat until stiff peaks form.

In another bowl, whip the cold cream and vanilla extract until it just holds soft peaks.

Stir the egg yolks into the cooled chocolate mixture until fully combined.

Using a rubber spatula, gently fold the whipped cream into the chocolate mixture until just blended.

Fold in one-third of the beaten egg whites to lighten the mixture, then carefully fold in the rest until no white streaks remain. Be careful not to overmix.

Spoon the mousse evenly into 4 serving dishes or glasses. Cover and chill in the refrigerator for at least 2 hours, or until set.

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