Pat the chicken dry, then place chicken breasts on a cutting board; if desired lay a sheet of plastic wrap over the top to keep things tidy. With a meat mallet or rolling pin, gently pound the breasts into an even thickness. Place in a sturdy ziptop bag or a small dish that will hold it snugly.
In a small bowl, stir together the marinade ingredients: ketchup, maple syrup, vinegar, Worcestershire, smoked paprika, cumin, salt, garlic powder, and black pepper.
Add the marinade to the chicken (either the bag or the dish). Seal the bag (if using), squeezing out as much air as possible. Turn to coat the chicken in the marinade. Marinade the chicken in the refrigerator for at least 2 hours or up to 1 day.
When ready to cook, remove the chicken from the refrigerator and let stand at room temperature while you preheat the oven. Place a rack in the center of the oven and preheat to 425°F. Line a rimmed baking sheet or baking dish large enough to hold the chicken without it touching with parchment paper.
Remove the chicken from the marinade and place on the parchment paper. Bake BBQ chicken for 14 to 16 minutes (for small/medium breasts that are about 6 to 7 ounces), 16 to 20 minutes (for medium/large breasts that are 8 to 10 ounces), or 20 to 25 minutes (for larger breasts). For best results, use an instant-read thermometer. When the breasts reach 165°F, they are done. Note that the chicken’s temperature will rise as it rests; I prefer to remove mine at 155°F. If desired, when the chicken has about 5 minutes left, brush it liberally with BBQ sauce.
Transfer the chicken to a plate or cutting board. Cover and let rest for a minimum of 5 minutes. Enjoy hot with a side of barbecue sauce as desired.
TO STORE: Refrigerate baked BBQ chicken breast in an airtight container for up to 3 days.
TO REHEAT: Leftover chicken can be reheated in a baking dish in the oven at 350°F.
TO FREEZE: Freeze leftovers in an airtight freezer-safe storage container for up to 3 months. Thaw overnight in the refrigerator, then reheat according to the directions above.