Pat the cod fillets dry with paper towels on both sides. Trim any ragged edges and check for bones; remove with tweezers if needed. Let rest at room temperature for 10 minutes while you make the rub.

Make the blackening spice rub

In a small bowl combine paprika, onion powder, garlic powder, dried thyme, dried oregano, cayenne pepper, ground black pepper, and kosher salt. Mix thoroughly so the spices are evenly distributed.

Season the fillets

Brush each fillet lightly with olive oil (about 1/4 tbs per fillet) to help the rub adhere. Sprinkle the blackening rub generously and evenly over the top and sides of each fillet, pressing gently so it sticks. Use about 1 to 1.5 tbs rub per fillet depending on size and spice preference.

Heat the pan

Place a heavy skillet (cast iron preferred) over medium-high heat. Add the butter and let it melt and begin to foam; swirl the pan so the butter coats the surface. If the butter begins to burn, reduce heat slightly; you want a very hot pan but not charred butter.

Sear the fillets

When the butter is hot and foaming, add the fillets presentation-side down (the side with the most seasoning exposed) without crowding the pan. Cook undisturbed for 3 to 4 minutes until the spice crust is dark brown and crisp. Use a thin spatula to gently lift a corner to check color.

Flip and finish

Carefully flip each fillet and spoon some of the melted butter from the pan over the top (basting) for an even finish. Cook 2 to 4 more minutes on the second side until the fish is opaque and flakes easily with a fork; internal temperature should be 135–140°F (57–60°C) for moist cod. Thicker fillets may need the full time.

Rest

Transfer the cooked fillets to a warm plate and let rest 2–3 minutes; this lets juices redistribute and finishes carryover cooking.

Serve

Serve the blackened cod hot with lemon wedges for squeezing over the top and chopped parsley sprinkled as garnish.

Optional garnish / sauce

Quick lemon aioli: In a small bowl whisk together mayonnaise, lemon juice, chopped parsley, and hot sauce (if using). Taste and adjust salt or lemon as needed. Spoon a dollop alongside each fillet or drizzle lightly over the fish.