Take the steak out of the fridge at least forty minutes before it meets heat. A cold centre means the outside overcooks while the middle catches up.
Pat every surface completely dry with kitchen paper. Water is the enemy of a crust, because the pan wastes its energy boiling it off instead of browning the meat.
Salt generously on both sides, either right now or forty minutes ago. Not ten minutes ago.
Heat a dry cast iron pan until it is genuinely, slightly alarmingly hot. Then add the dripping.
Lay the steak away from you and do not touch it for two minutes. Not to look, not to reposition, not to check.
Turn, another two minutes, and add the butter at this point if you are using it. Tilt the pan and spoon the foaming butter over the top.
Stand it on its fat edge for thirty seconds to render the strip. Everyone forgets this and everyone should not.
Pull at 52C in the centre for medium rare. It will climb another three or four degrees while it rests.
Rest on a warm plate for five minutes minimum. Cut it now and the juice goes on the board instead of staying in the meat.