Preheat oven to 425 degrees.With a round cookie cutter, cut circles in the puff pastry and place each round in a mini muffin pan.
Bake for 10-15 minutes or until golden brown. The puff in the puff pastry will definitely be in action and will probably puff up in the pan. With a knife, carefully push the puff down. Allow to cool and remove from pan.
In a separate bowl, mix together mozzarella cheese, tomatoes, and basil. Toss to coat with olive oil and season with salt to taste.
Spoon tomato mixture into the cooled puff pastry cup.Drizzle with balsamic reduction.