To a large bowl, add chicken breasts and ¼ cup Balsamic Dressing. Completely coat the chicken with the dressing. Cover the bowl with plastic wrap and store in the refrigerator for one hour.
Preheat the oven to 400 degrees. Heat a large oven-safe sauté pan to medium-high on the stove. Add olive oil to the pan. Once the oil is hot and shiny, add chicken breasts. Season with salt and pepper to taste. Sear each side of the chicken for about three minutes until golden brown on the outside.
Move chicken to the oven and bake it for about three minutes on each side until the internal temperature of the chicken reaches 165 degrees and it’s fully cooked throughout.
Set the oven broiler to high. Add a layer of fresh mozzarella to each piece of chicken breast and broil the chicken for about three minutes until the cheese is completely melted. Transfer chicken to a cutting board to rest for 10 minutes.
Heat a large pan to medium on the stove and add butter. Once butter melts, add the brioche buns, insides facing down. Toast the buns about one minute on each side until golden brown. Transfer to a cutting board.
Spread the remaining Balsamic Dressing on the inside of the toasted buns. Assemble sandwiches in this order: bottom bun, slice of tomato, balsamic chicken with melted mozzarella, fresh basil, top bun.
