Prepare the chicken: Place the chicken breasts on a cutting board. If the chicken breasts are very thick, carefully butterfly them by slicing horizontally through the center, stopping before cutting all the way through, and opening them like a book. Cover with parchment paper or plastic wrap and use a meat mallet or rolling pin to gently pound the chicken to an even thickness of about 1/2 to 3/4 inch. If the chicken breasts aren't too thick to begin with, simply pound them until they are an even thickness. This helps the chicken cook evenly and stay juicy.

Make the marinade: In a large bowl or zip-top bag, whisk together the balsamic vinegar, olive oil, Dijon mustard, sea salt, garlic powder, Italian seasoning, and black pepper.

Marinate the chicken: Add the chicken and toss to coat well. Cover and refrigerate for at least 2 hours and up to 24 hours.

Preheat the grill: Heat the grill to medium-high heat, about 400 to 450°F, and lightly oil the grates.

Grill the chicken: Remove the chicken from the marinade, allowing any excess to drip off. Grill for 5 to 7 minutes per side, depending on thickness, flipping once halfway through cooking.

Check the temperature: Using an instant-read meat thermometer, check the temperature in the thickest part of the chicken. Remove the chicken from the grill when it reaches 165°F. Don't skip the meat thermometer — it's the easiest and most reliable way to ensure perfectly cooked, juicy chicken every single time.

Rest and serve: Transfer the chicken to a plate and let it rest for 5 minutes before slicing and serving.