Prepare the glaze: in a sauce pan combine 1 cup balsamic vinegar, ¾ cup ketchup, ⅓ cup brown sugar, 1 tablespoon each of Worcestershire sauce and dijon mustard and long with ½ teaspoon each of salt, black pepper and garlic powder. Stir to combine then simmer for 15-20 minutes, or until the sauce has reduced by about ⅓ . Reserve ⅓ of the sauce for dipping.
Prepare the chicken: while the balsamic BBQ sauce glaze simmers, prepare the chicken. For the best results, pound the chicken breasts to an even thickness or butterfly them to ensure even cooking. Drizzle the chicken with olive oil, then generously season with salt, black pepper and garlic powder.
Preheat the grill or grill pan: Heat your grill or grill pan to medium-high heat.
Cook the chicken: Place the chicken on the grill. Use a pastry brush to brush each piece of chicken with some of the balsamic bbq sauce. Cook for about 5-7 minutes then flip the chicken.Brush the chicken with more balsamic glaze and continue cooking for another 5-7 minutes, or until the internal temperature of the chicken is 165 degrees F. As the chicken is cooking, brush each side with sauce every few minutes. Rest and Serve: Once the chicken is cooked through, allow to rest for a few minutes before serving. Slice the chicken against the grain to keep it tender. Be sure to serve the remaining warm balsamic bbq sauce for dipping.