Preheat oven to 400°.
Place bread in a food processor, and process until finely ground to measure 1/2 cup. Set aside.
Core tomatoes; cut each tomato in half crosswise. Push seeds out of tomato halves with thumbs. Spoon 1 tablespoon buttermilk into each tomato half. Combine breadcrumbs, cheese, pepper, and egg white in a small bowl. Divide breadcrumb mixture evenly among tomato halves. Place tomato halves on a baking sheet coated with cooking spray. Bake at 400° for 17 minutes.
Broil tomato halves 2 minutes or until lightly browned. Sprinkle each tomato half with 1 teaspoon cilantro.