Heat Balsamic vinegar over medium heat in a heavy bottomed sauce pan, stirring occasionally. Continue to simmer for 15-25 minutes, or until reduction thickens enough to coat a spoon. Remove from heat and allow to cool.
Meanwhile, begin to assemble cracker stacks. Start by topping each cracker with a slice of mozzarella, followed by a slice of tomato, a basil leaf, and salt and pepper.
Right before serving, drizzle each cracker stack with balsamic reduction. Serve immediately. Enjoy!