Thaw frozen corn and pat dry with paper towels. Preheat oven to 450 degrees F. Line a 15x10x1-inch baking pan with foil. Lightly grease the foil. Spread corn on half of the prepared pan. In a resealable plastic bag, combine potatoes and 1 tablespoon of the olive oil. Seal and shake well to coat potatoes with oil. Spread potatoes on the other half of the prepared pan. Roast, uncovered, for 10 minutes. Stir, keeping corn and potatoes separate. Roast for 10 minutes more, stirring once or twice. Remove pan from oven. Set aside.

Transfer half of the roasted corn (about 3/4 cup) to a food processor or blender. Cover and process or blend until corn is pureed (if necessary, add a small amount of chicken broth to help blend corn).

In a 4-quart Dutch oven, heat the remaining 1 tablespoon olive oil over medium heat. Add leek and shallot; cook and stir for 6 to 8 minutes or until leek is very soft and golden. Add whole corn and pureed corn. Cook and stir for 1 minute. Stir in roasted potatoes, the 4 cups chicken broth, marjoram, salt, ginger, and white pepper. Bring to boiling; reduce heat. Cover and simmer for 10 to 12 minutes or until potatoes are tender.

Add half-and-half. Cook and stir until heated through. Season to taste with additional salt and white pepper.

Hollow out the sourdough loaves. To serve, spoon the chowder into bread bowls.