In a medium bowl, whisk together the whiskey, brown sugar, vinegar, Worcestershire sauce, and garlic.
Pour the marinade into a sealable bag, add the chicken thighs to the bag and seal it shut. Place in the refrigerator either in a bowl or with a paper towel underneath the bag in case of leakage. Allow the chicken to marinate at least an hour.
While the chicken is marinating, heat the grill to medium high. Spray the grill with oil. Before placing the chicken on the grill, lower the heat to medium.
Remove chicken from the marinade and transfer remaining marinade to a small pot.
Grill chicken for approx. 10 minutes on each side, total cooking time approx. 20 mins. An instant-read thermometer when inserted into the chicken should read 165 degrees. (Note: Because the marinade has a lot of sugar in it, the grill can have flare-ups. It's important to lower the temperature and move the chicken around if this happens to avoid burning the chicken.)
While chicken is cooking, heat the reserved marinade over medium-low heat. Keep an eye on it to avoid having it boil over. Keep the marinade at a full simmer for 5-7 minutes to burn off some of the booziness from the alcohol, then add the cornstarch slurry one teaspoon at a time (you may not need to add the full tablespoon), making sure to keep mixing until desire sauce thickness is reached.
Place chicken on a platter, spoon over bourbon sauce, and garnish as desired. Dig in and serve with extra napkins😉!