Hallow out top and bottom halves of bread, leaving a 1/4-inch-thick shell. Place torn bread in a food processor; process until very fine crumbs form. Set aside 1/2 cup breadcrumbs; reserve remaining breadcrumbs for another use. Combine 1/2 cup breadcrumbs, cornmeal, and black pepper in a large zip-top plastic bag; seal and shake to combine.