Preheat the oven to 425°F (218°C) and line a baking sheet with parchment paper or foil; lightly spray with cooking oil if desired.
Pat the chicken thighs dry with paper towels to remove excess moisture; drying helps the skin crisp.
In a bowl, combine the baking powder, dry ranch seasoning, garlic powder, and black pepper until evenly mixed.
Rub the spice mixture evenly over the skin and exposed surfaces of each chicken thigh.
Arrange the thighs skin-side up on the prepared baking sheet, leaving space between pieces for air circulation.
Bake at 425°F for 30–40 minutes; if using a convection or air-fryer oven setting, cook toward the lower end of that range. If using boneless thighs, begin checking for doneness after about 25 minutes.
Cook until the internal temperature of the thickest part reaches 165°F (74°C) and the juices run clear; baste with pan juices if desired.
Optional: For extra crispness, place the thighs under the broiler for 1–2 minutes, watching closely to prevent burning.