Place the balsamic vinegar and brown sugar in a small saucepan and bring to a boil. Reduce heat and let simmer for 7-10 minutes, or until reduced by half and thickened slightly. Pour into a small bowl or cup and let cool to room temperature.
Pound out both chicken breasts to about ⅓" thickness. Coat with olive oil, salt, and pepper and grill on both sides until cooked through. Place on a plate and let rest for 5 minutes. Slice each breast into 2-3 pieces.
To assemble: On a platter, layer slices of chicken, tomato, and cheese. Drizzle olive oil and balsamic reduction over the top. Sprinkle coarse ground pepper and salt on top to taste. Finish with a sprinkling of fresh basil leaves.
Recipe by The Kitchen McCabe at https://www.thekitchenmccabe.com/2015/08/25/grilled-chicken-caprese-salad/