In a small bowl, mix together cocoa powder and hot water. Set aside to cool slightly.
In a double boiler, melt chocolate. Cool 3 to 5 minutes.
In a large mixing bowl of a stand mixer, whip cream, sugar and salt at medium speed. Whip for 30 seconds. It should slightly thicken. Increase the speed to high and whip the cream until soft peaks form. It should happen quite fast ( 20 to 30 seconds ) so do not walk away. Set aside.
Whisk cocoa powder mixture with melted chocolate until smooth. At first it will look like your chocolate seized but keep whisking. It will come back together and be smooth.
With whisk, fold in ⅓ of the whipped cream into chocolate mixture to lighten it. Switch to a rubber spatula and gently fold in the rest of the whipped cream, until no streaks remain. It is important or the mousse won't set right.
Spoon the mousse into glasses.
Refrigerate until ready to serve or for up to two days.