Scoop out the flesh of the avocado, and place it a food processor. Add in the cacao powder, sweetener, vanilla and 1/4 teaspoon salt and process until completely pureed and aerated. No pieces of avocado should be visible, if there are any, turn processor off and scrape its walls with a silicone spatula. Process again, until completely creamy and smooth. Test the saltiness and add 1/4 teaspoon, if needed.