1. Hollow out top and bottom halves of bread, leaving a 1/2-inch-thick shell; reserve torn bread for another use. Spread 2 tablespoons mayonnaise over cut side of each bread half. Sprinkle basil and 1/2 cup cheese on bottom half of loaf. Top evenly with prosciutto, tomato slices, and remaining 1/2 cup cheese. Cover with top half of loaf. Cut filled loaf crosswise into 4 equal pieces.
2. Heat a grill pan over medium heat. Coat pan with cooking spray. Add sandwiches to pan. Place a cast-iron or other heavy skillet on top of sandwiches; press gently to flatten sandwiches. Cook 3 minutes on each side or until bread is toasted (leave cast-iron skillet on sandwiches while they cook).