Bring balsamic vinegar up to a boil in a saucepan, then lower heat to medium and simmer until it’s the consistency of very thin maple syrup, about 10 minutes. Pour into a bowl and let cool.
Thread a piece of mozzarella, basil leaf and a tomato onto a tooth pick. Repeat with remaining ingredients. Sprinkle with salt & pepper, then drizzle cooled balsamic reduction on top.
