Place the chopped dark chocolate in a medium heatproof bowl. Pour the hot water over the chocolate and let it sit for 2 minutes. Whisk until the chocolate is completely smooth and melted. Stir in the vanilla extract and salt. Set aside to cool slightly.

In a separate small bowl, whisk the three egg yolks with half of the sugar (2 tablespoons) until the mixture is pale yellow and slightly thickened. Whisk the yolk mixture into the cooled chocolate mixture until fully combined.

In a clean, grease-free bowl, beat the three egg whites with an electric mixer until soft peaks form. Gradually add the remaining 2 tablespoons of sugar while continuing to beat until stiff, glossy peaks form.

In a third bowl, whip the cold heavy cream until medium-stiff peaks form.

Gently fold one-third of the whipped cream into the chocolate mixture to lighten it. Then, gently fold in the remaining whipped cream until just combined, being careful not to deflate the air.

Gently fold the egg white meringue into the chocolate mixture in two additions, using a large spatula. Fold until no white streaks remain, keeping the mixture light and airy.

Divide the mousse into individual serving dishes or ramekins. Cover and chill in the refrigerator for at least 4 hours, or until firm.