Preheat the oven to 400 degrees Fahrenheit (200 degrees Celsius).
Rinse off your cod fillets in water and pat dry with a paper towel. Set aside.
Mix together the paprika, salt, garlic powder, oregano, thyme, black pepper, onion powder, cayenne pepper and red pepper flakes and set aside.
Coat the bottom of the oven save baking dish with half of the melted butter and add in your cod fillets.
Sprinkle half of Cajun spice mix to one side of the cod fillet and rub in coating the entire surface. Turn over the fillet.
Sprinkle the rest of the Cajun spice mix on the other side of the fillet and rub in, coating the entire surface.
Place the squares of butter on top of the fillets, evenly spaced out. This is optional.
Bake in the oven for 20-25 minutes, depending on the thickness of your fish. Remove from the oven once it is done and serve with your favorite side dish!