Combine the brown sugar, paprika, garlic powder, onion powder, salt, and pepper.
Pat chicken thighs dry with a paper towel and season the chicken thighs all over with the spice mix.
Coat the inside of a slow cooker with cooking spray.
Place the thighs in the slow cooker. Pour 2 cups of BBQ sauce into the slow cooker and toss to coat.
Cover and cook on high for 3 to 4 hours or low for 5 to 6 hours. When done cooking, spray a sheet pan with cooking spray and pre-heat the broiler.
Remove the chicken thighs from the slow cooker and place them on a baking sheet in a single layer. Brush the remaining ½ cup bbq sauce over the thighs.
Broil for 3 to 5 minutes or until the chicken has browned and crisped up.