1. Puree onion in a blender or food processor along with enough tomatoes to yield about 4 cups of tomato puree.
2. Pour puree into a 4- to 5-quart slow cooker along with broth, brown sugar, basil, Worcestershire, and cocoa powder. Cover and cook on Low 6 to 8 hours.
3. Ten minutes before serving, stir in evaporated milk and butter; season with salt and pepper. Heat an additional 10 minutes, and serve.