Balsamic vinegar is good friends with buffalo mozzarella. The tanginess of the vinegar is a nice complement to the subtlety of the cheese. Because this recipe is a little bit sweet and a little bit savory it can be served as a appetizer or dessert.
Put vinegar in a medium saucepan and bring to a boil over medium heat. Adjust heat to low and simmer until it is reduced nearly 3/4 in volume and thickened to a syrup. About 15 minutes.
Add plums to syrup, turning gently to coat and continue to cook for another 4-7 minutes more. Turn off heat and let cool.
Meanwhile, assemble mozzarella slices on platter, allowing to overlap slightly and season with salt and pepper to taste. With a fork, remove plums from syrup and place atop cheese, then top with the remaining syrup, pine nuts, and mint. Drizzle with olive oil and serve.