Brush both sides of the chicken breast or fish fillet with butter, then cover liberally with blackening seasoning.
Heat a cast-iron skillet over high heat for 10 minutes, or until smoking hot. Pour half of the remaining melted butter into the skillet. It will smoke, so turn on the vent fan!
Lay the meat on top of the butter. Let the meat sear for 3 minutes without moving, then flip.
Pour the remaining butter onto the top of the meat, then sear for 2 minutes more.