In a small saucepan, bring balsamic vinegar to a boil over medium-low heat. Cook for 10 to 20 minutes, or until balsamic has reduced to a thicker glaze. Remove from heat and transfer to a bowl or cruet. Allow to cool.
Arrange tomato and mozzarella slices on a platter, put basil leaves between the slices. Drizzle olive oil over the top of the salad, getting a little bit on each slice. Do the same with the balsamic reduction. Sprinkle salt and freshly ground black pepper.
Store extra balsamic reduction in fridge for a later use.