Set oven rack about 6 inches from the heat source and preheat the oven’s broiler. Line a baking sheet with aluminum foil and spray with cooking spray.
Arrange haddock fillets on the prepared baking sheet. Mix onion powder, paprika, garlic powder, salt, black pepper, and cayenne pepper in a small bowl; sprinkle seasoning over haddock. Dot haddock with pieces of butter.
Broil in the preheated oven until fish is easily flaked with a fork, 6 to 8 minutes. Serve with lemon wedges.