In a large saucepan, melt butter over moderately low heat. Add scallion bulbs, bell pepper, and celery. Cook, stirring occasionally, until vegetables start to soften, about 10 minutes.

Stir in potatoes, 2 cups corn, bay leaf, broth, and salt. Bring to a boil. Reduce heat and simmer, stirring occasionally, for 15 minutes.

In a blender or food processor, puree remaining 2 cups corn with milk.

Stir puree into soup along with black pepper. Simmer until soup thickens slightly, 5 to 10 minutes.

Remove bay leaf. Stir in scallion greens. Top each serving with a dollop of sour cream, if using.