Corn Chowder

Name: Corn Chowder
Ingredients: 2 tablespoons butter; 4 scallions, white bulbs and green tops chopped and reserved separately; 1 red bell pepper, chopped; 4 ribs celery, chopped; 1 pound (about 3) boiling potatoes, peeled and cut into 1/2-inch dice; 4 cups fresh corn kernels (cut from 6 to 8 ears), divided; 1 bay leaf; 1 (32-ounce) can low-sodium chicken broth or homemade stock; 2 teaspoons kosher salt; 2 cups milk; 1/4 teaspoon freshly ground black pepper; Sour cream, for serving (optional)