Warning: You must do this recipe outside or under a commercial vent hood. There is risk of fire. To get that blackened look and taste, you must fire up the skillet to near rocket-hot. To get the skillet that hot, you have to have the stove high enough that it could cause a flame up when the butter hits the pan. Just be careful.
When I cooked mine, I too the surface temperature of the skillet at 550 degrees.
On a grill or other type of stove outdoors, heat an iron skillet to rocket hot. This skillet cannot be hot enough. In a small bowl, mix the spices together. Rub generously on the fish fillets.
Melt 3/4 of the stick of butter in a small glass or ceramic dish. Dip the fillets in the melted butter then take them directly to the hot skillet. As soon as the fish goes down, put 1 TBSP of the remaining butter on each fillet.
Cook for 2 minutes on the first side, then, using a spatula, flip each fillet over and cook another 2 minutes. That should be all it takes to get this done.