Heat 2 tablespoons of the olive oil in a large heavy-based frying pan. Season the cod and add to the pan, skin side down. Cook for 2 to 3 minutes, until the skin is beginning to crisp. Add the butter to the pan and cook for another 2 minutes, until the skin is nice and crisp. Turn the cod over and cook for a further 3 to 4 minutes, until cooked through. Transfer the cod to a warm plate and keep warm.