Heat butter and oil in a large Dutch oven over medium heat. Add leek, celery, and bell pepper; cook 4 minutes or until vegetables are tender, stirring frequently. Combine milk and flour in a small bowl, stirring with a whisk. Slowly add milk mixture to pan, stirring constantly. Stir in broth, corn, potato, salt, and freshly ground black pepper; bring to a boil. Reduce heat, and simmer 20 minutes or until potato is tender. Stir in parsley and chives.