Pour the coconut milk and sweetener into a small saucepan, and heat over medium-high heat until begins to a simmer. Whisking until it's completely dissolved.
Add the cocoa, and salt and mix it up. Allow cooling completely.
In a separate mixing bowl, whip the heavy whipping cream until stiff peaks form.
Place the whipped cream and vanilla extract in the chocolate mixture and gently fold.
Place in a serving glass and garnish with heavy whipping cream if you want.
Notes
The mousse is best served after chilled for at least an hour.