Preheat the panini grill or heat a skillet over medium heat.
In the meantime, place 1 bread slice on a work surface. Arrange basil leaves, tomato slices, and mozzarella cheese on the bread. Cover with the remaining bread slice.
Lightly coat the panini grill or skillet with cooking spray before placing the sandwich.
Cook for 2 minutes on each side of the sandwich or until golden brown and the cheese is melted. Cut panini in half.