Put 2 cups of the corn and the milk into a blender or food processor, until smooth. Set aside.
Heat the oil in a large soup pot over a medium-high heat. Add the onion, bell pepper, and zucchini and cook, stirring until the vegetables are tender, about 5 minutes. Add the remaining 2 cups of corn and the broth and bring to a boil. Add the pureed corn and tomatoes and cook until warmed through, but not boiling. Add the salt and season with pepper. Serve garnished with the basil ribbons.