Preheat the oven to 400°F (200°C). Line a baking sheet with foil and lightly coat with nonstick cooking spray.
In a small bowl, whisk together the paprika, kosher salt, garlic powder, onion powder, cumin, cayenne, black pepper, and brown sugar until evenly combined.
Pat the chicken thighs dry with paper towels. Rub the seasoning mixture under the skin and all over the outside of each thigh.
Arrange the seasoned thighs skin-side up on the prepared baking sheet, spacing them evenly.
Brush each thigh evenly with half of the BBQ sauce. Bake for 18–20 minutes.
Remove the pan, brush the thighs with the remaining BBQ sauce, then return to the oven and bake for about 20 more minutes, or until the internal temperature reaches 165°F (74°C).
Switch the oven to broil and broil the thighs 2–4 minutes, watching closely, until the sauce caramelizes and edges are slightly charred.
Let the chicken rest for 5 minutes, garnish with chopped green onions, and serve with extra BBQ sauce if desired.