1. In a small saucepan, bring the balsamic vinegar to a boil. Reduce heat and simmer until it has reduced by half and is thickened, about 10-15 minutes. Set aside to cool.
2. Arrange tomato and mozzarella slices on a platter, alternating and overlapping them. Tuck basil leaves in between the slices.
3. Drizzle the olive oil over the salad, then drizzle the balsamic reduction. Season with salt and pepper to taste.