In a large bowl, whisk together the soy sauce, Dijon mustard, lemon juice, Worcestershire sauce, balsamic vinegar, and olive oil until well combined. Add the brown sugar, garlic powder, dried rosemary, salt, and black pepper, stirring until the sugar dissolves completely and all ingredients are fully incorporated. Before adding the chicken, carefully measure out 1/2 cup of this marinade into a separate small bowl and set it aside in the refrigerator—this reserved portion will be used for basting later and ensures you have raw marinade for brushing without cross-contamination concerns. I prefer to reserve the basting liquid before the chicken touches the marinade, which gives me peace of mind about food safety.

Place the pounded chicken breasts into a large resealable plastic bag or shallow dish. Pour the marinade (minus the reserved portion) over the chicken, ensuring all pieces are evenly coated. Seal the bag or cover the dish, then refrigerate for at least 30 minutes, though ideally 4-8 hours or up to 24 hours for deeper flavor development. The longer marination time allows the acidic ingredients to tenderize the chicken while the spices penetrate the meat.

About 10-15 minutes before you're ready to cook, preheat your grill to medium-high heat (around 375-400°F if using a gas grill, or prepare charcoal for similar temperature). Once the grill is hot, lightly oil the grates using a folded paper towel dipped in oil and held with tongs—this prevents the chicken from sticking and helps create attractive grill marks. Remove the chicken from the refrigerator and let it sit at room temperature for about 5 minutes while the grill finishes heating; this ensures more even cooking throughout the meat.

Remove the chicken breasts from the marinade, allowing excess liquid to drip back into the bag (discard this marinade). Place the chicken on the hot grill and cook for 5-6 minutes without moving them, which allows proper browning and grill marks to develop. Flip the chicken, then cook the second side for another 5-6 minutes, basting with the reserved marinade from Step 1 every minute or two during this final phase. I like to baste frequently during the last few minutes of cooking as it caramelizes beautifully and adds an extra layer of flavor, but be careful not to brush with raw marinade that touched the raw chicken.

Continue grilling until the chicken reaches an internal temperature of 165°F when measured at the thickest part with an instant-read thermometer. Transfer the cooked chicken to a clean plate and loosely tent it with aluminum foil, then let it rest for 5 minutes before serving. This resting period allows the juices to redistribute throughout the meat, ensuring tender, moist chicken rather than one that's dry from losing all its internal moisture immediately.