In a large, ovenproof skillet, combine 2 tablespoons of the olive oil, the butter, and garlic. Warm over medium heat until the butter melts.

Add bread cubes to the skillet and toss to combine. In an oven preheated to 350° F, bake until golden, about 15 minutes, tossing several times during baking. Remove from oven and let bread cool in the pan.

In a large bowl, combine remaining olive oil, balsamic vinegar, salt, and pepper. Mix gently to combine. Stir in bread, tomatoes, about 3/4 of the basil, and mozzarella. Season with additional salt if needed.