Place the avocado, cocoa powder, water, vanilla, sweetener, and salt in your food processor bowl (I use a mini food processor).
Process until smooth and creamy, for about one minute, stopping once to scrape the sides of the bowl with a spatula if needed.
When done, the mousse should be thick and creamy. Taste it and add more cocoa powder, sweetener, or water as needed.
Allow the mousse to rest at room temperature for 30 minutes, then serve.
Video
Notes
If your avocado is significantly larger than the one I typically use, try using ½ cup of cocoa powder and ½ cup of water, 2 teaspoons of vanilla, and 2 teaspoons of stevia glycerite (to equal ⅔ cup of sugar), then taste and adjust.
You can make the mousse a few hours ahead and refrigerate until ready to eat, but remove it from the fridge 30 minutes before enjoying it to allow it to return to room temperature. Chocolate always tastes better at room temperature.
The avocado needs to be Haas avocado and very ripe; the cocoa powder should be Dutch-processed (processed with alkali); and it's important to let the mousse rest for 30 minutes to allow the flavors to meld.
Because it contains raw avocado, I suggest you make this mousse on the day you plan on eating it. You can make it a few hours in advance and refrigerate it covered. But I wouldn't try to keep it any longer than that. I don't recommend freezing this mousse.