In a medium bowl, whisk together the 1/2 cup balsamic vinegar, 1/2 cup water, 1/4 cup olive oil, 1/4 cup dark brown sugar, minced 3-4 cloves fresh garlic , minced 1 tablespoon fresh rosemary, 1/2 teaspoon ground black pepper and 1/2 teaspoon coarse sea salt until the sugar dissolves.
Place the 1.5 pounds boneless skinless chicken breasts in a shallow baking dish or pan. Remove any excess fat and pat dry with paper towels. Pour the balsamic marinade over the chicken, turning over to ensure all sides are coated with the glaze.
Preheat the grill to medium-high heat (around 350°F) and oil the grates.
Cook on indirect heat for 20-25 minutes, turning, occasionally, or until internal temperture of chicken reaches 165°F. Cooking times will vary greatly depending on the thickness of the chicken breasts.
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