Coat one side of each bread slice with cooking spray. Place bread slices on a work surface, coated sides down. Arrange tomatoes, mozzarella cheese, and basil on four of the bread slices. Cover with the remaining four bread slices, coated sides up; press together gently.
Preheat a panini grill or heat a large skillet over medium heat. Add sandwiches to hot panini grill or skillet; cook for 3 to 4 minutes or until golden brown and cheese is melted, turning once if using a skillet. Serve immediately. If desired, serve with balsamic vinegar for dipping.