Add the avocados, cocoa powder, maple syrup, milk, and vanilla extract to a blender or food processor.

Blend on high speed for 1-2 minutes, stopping to scrape down the sides as needed, until completely smooth and creamy.

Taste and adjust sweetness by adding more maple syrup if desired, then blend again briefly to incorporate.

Divide the mousse among four serving dishes and refrigerate for at least 1 hour before serving to allow the flavors to meld and the texture to firm up slightly.