How to Make It
In a large saucepan, melt the butter over moderately low heat. Add the scallion bulbs, bell pepper, and celery and cook, stirring occasionally, until the vegetables start to soften, about 10 minutes. Stir in the potatoes, 2 cups of the corn, the bay leaf, broth, and salt. Bring to a boil. Reduce the heat and simmer, stirring occasionally, for 15 minutes.
In a blender or food processor, puree the remaining 2 cups corn with the milk. Stir the puree into the soup along with the black pepper. Simmer until the soup thickens slightly, 5 to 10 minutes. Remove the bay leaf. Stir in the scallion greens. Top each serving with a dollop of sour cream, if using.
If you want to use frozen corn, puree two cups of it with the milk as directed above, and add the remaining two cups to the soup along with the puree. Since the corn is already cooked, it might toughen if it goes in earlier. You could add a pinch of sugar, too.