In a medium bowl, whisk together the vinegar, olive oil, brown sugar, garlic, pepper and garlic salt. Reserve about 3 tablespoons of the glaze in a small bowl and set aside.
Place the chicken thighs in a shallow baking dish or pan. Remove any excess fat and pat dry with paper towels. Pour the balsamic glaze over the chicken, turning over to ensure all sides are coated with the glaze.
Preheat your air fryer to 390 degrees. (Check your owners manual to see exact preheat times for your model of air fryer. Preheat times are typically 3-5 minutes.)
To the preheated air fryer basket, place the glazed chicken, skin side down. Discard the glaze that was in the baking dish. Evenly space the chicken thighs, making sure to place in one layer. Cook the chicken for 10 minutes at 390 degrees.
After 10 minutes, turn the chicken thighs over, and brush with the reserved balsamic glaze that was previously set aside. Cook the chicken for an additional 10 minutes.
Once done cooking, let the chicken rest for 3-5 minutes before serving.
Notes
Want to double the recipe? I recommend cooking the chicken thighs in two separate batches. Depending on the size of your air fryer, you'll have the best results by cooking 4-5 thighs at once. I don't recommend overcrowding or stacking more than 5 thighs into the air fryer basket at one time. The chicken will not cook evenly.