Add the almond milk, chia seeds, sweetener, cocoa powder, vanilla extract, and salt to a blender.
Start blending on a low speed, then slowly increase to a higher speed. This helps everything mix smoothly.
Blend until the mixture becomes thick and creamy, and the chia seeds are barely noticeable. Stop if needed to scrape down the sides so everything blends evenly.
Once smooth, the mousse is ready to eat right away. Add your favorite toppings and serve.
For a colder and firmer texture, transfer the mixture to a sealed container and place it in the fridge for about 3 to 4 hours before serving.