Add all ingredients to a large bowl or gallon ziploc bag and toss to combine. Marinate for at least 30 minutes at room temperature, or up to 24 hours in the refrigerator. Don’t exceed 24 hours, the acid in the balsamic will start to make the chicken a funky texture.
Preheat grill or oven to 450°F.
Pour the marinade into a small pot and bring to a boil over high heat. Continue cooking, stirring often, over medium-high heat until it has reduced by half to a syrup consistency, 5 to 7 minutes.
Shake as much marinade off the chicken thighs as possible, then:
To grill: place on the grill and insert a probe into the center of the smallest thigh. Place skin-side-up on the grates and grill for 15 to 20 minutes, until the internal temperature reads 165°F.
To roast: place on a parchment-lined rimmed baking sheet and bake for 15 to 20 minutes, until the internal temperature reads 165°F.
Brush the grilled chicken breasts all over with the reduced marinade. Allow chicken to rest for at least 5 minutes before serving to allow the juices to distribute evenly throughout the meat.