Place the chopped chocolate, cocoa powder, salt, and one (1) cup of the heavy whipping cream into a medium saucepan over medium-low heat. Cook, stirring constantly, until the mixture has melted and is smooth and glossy. Remove from the heat and stir in the vanilla extract. Allow to cool completely to room temperature.
Once the chocolate mixture is at room temperature, add the remaining heavy whipping cream and the confectioners' sugar to the bowl of a stand mixer fixed with the whisk attachment. Whip the mixture on high speed until stiff peaks form, about 5-7 minutes.
Fold 1/3 of the whipped cream mixture gently into the chocolate mixture, taking care not to deflate the whipped cream. Repeat twice more, working in stages, until the mixture is fully incorporated and no streaks of whipped cream or chocolate remain.
At this point, you can distribute the mousse among individual cups or ramekins, or place it in a larger bowl for serving. Cover the serving vessels of your choice with plastic wrap or foil and place in the fridge to set for at least 6 hours, or overnight.
Just before serving, garnish with fresh whipped cream and chocolate shavings, or fresh fruit, if desired.