Start by sprinkling the salt (not the sea salt) evenly all over the steak, i.e. a dry brine, the day before. Keep it in a plastic bag in the fridge.
Take it out and apply half the flaked sea salt on the “inside”. Truss it with some butcher's twine. Rub the outside with the other half of the flaked sea salt.
Prepare your grill for indirect heat, have it running at 120° C / 250° F. Place the steak on the cooler side, with a thermometer in the meat. Cook it until the internal temperature of the meat reaches 52° C / 125° F.
Now move it to the hotter side for searing. Sear it on all sides, rotate it a few times, and remove it when the internal temperatures reaches 56-58° C / 133-136° F.
Wrap it in aluminum foil with the butter. Wrap it in a towel and let it rest for 10 minutes. Unwrap it, slice it, and serve, why not with a chimichurri!
In the oven
If you don’t want to cook on a grill, this can also be cooked in the oven. Set your oven to 120° C / 250° F and place your steak in the middle. When it reaches 52° C / 125° F, fry it in the frying pan until it reaches 56-58° C / 133-136° F, then do the wrapping as described above.