In a small heatproof bowl, microwave chocolate in 30-second increments, stirring between each, until melted and smooth, about 90 seconds total. Let cool slightly.
In a food processor or small blender, blend cottage cheese until smooth. Add chocolate and blend, scraping sides and bottom, until well combined. Transfer to a container with a tight-fitting lid. Refrigerate until well chilled, at least 1 hour and up to 3 days.
Top mousse with more chopped chocolate and berries (if using).
