Pour the balsamic vinegar into a very small saucepan. Heat over medium-low heat for about 10-15 minutes, keeping it at a low simmer but not boiling. Simmer until slightly thickened but still pourable, and volume has reduced to about ⅓ cup. (This is balsamic reduction - it will thicken as it cools, so don't let it thicken too much when hot.)
When the balsamic reduction is ready, let it cool to room temperature.
Meanwhile, in a large bowl, whisk together the olive oil, garlic powder, sea salt, and black pepper.
Add the tomatoes, mozzarella, avocado, and fresh basil. Toss to coat. Drizzle the entire salad with 1 tablespoon balsamic reduction. (You can save the rest for another use.)