1. Using a sharp knife, cut focaccia into 2 x 2 3/4 x 3/4-inch rectangles. Cut tomatoes into slices about 1/4 inch thick, then cut them in half again. If using cherry tomatoes, leave them whole.
2. To assemble skewers, stack a slice of bread, a slice of mozzarella, a basil leaf and a slice of tomato. Secure stack with 2 of the skewers. Repeat with remaining ingredients.
3. Preheat a grill to high heat. Brush and clean grill grid with a cloth, and lightly oil grill so that skewers will get a good sear but still release from grate. Grill for 1 1/2 to 2 minutes per side, or until bread is light brown and cheese is melted.
4. Remove Panzanella Skewers from grill, place on a platter and remove skewers. Drizzle with olive oil and serve.