Remove the excess fat and add your boneless chicken thighs to a large bowl. Add brown sugar, kosher salt, garlic powder, onion powder, smoked paprika, chili powder and avocado oil. Mix it all together so that each piece of coated. Place it in an airtight container and keep it in the fridge for at least 4 hours up to overnight.
Remove the marinated boneless skinless chicken thighs from the fridge. Let them come to room temperature, covered, for 30 minute before baking.
Meanwhile, preheat your oven to 400F degrees.
Add half of your bbq sauce to the boneless skinless thighs and toss until coated. Place chicken thighs in a single layer on an aluminum foil lined rimmed baking sheet - making sure that they're evenly spaced apart.
Bake the chicken for 20-25 minutes, or until the internal temperature of the chicken reaches 175-190 degrees (or 165 degrees F for white meat). Baking time may vary slightly depending on the thickness of the chicken thighs so use a meat thermometer.
Once cooked through, add more bbq sauce and place under the broiler for the last 2-3 minutes of cooking. Watch closely to prevent burning.
Once done, remove the BBQ chicken thighs from the oven and let them rest for a few minutes. This allows the juices to redistribute and the chicken to finish cooking through. Serve with the remaining BBQ sauce and enjoy!