Combine the brown sugar, paprika, chili powder, garlic powder, onion powder, salt, and pepper. Cayenne if using.
Pat chicken drumsticks dry with a paper towel. Season chicken all over with the spice mix.
Coat the inside of a slow cooker with cooking spray. Place the drumsticks 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 drumsticks from the slow cooker and place them on a baking sheet in a single layer. Brush the remaining ½ cup bbq sauce over the drumsticks.
Broil for 3-5 minutes or until the chicken has browned and crisped up.