Remove the steak from the refrigerator, season with kosher salt and allow it to come up to room temperature for 60 minutes. If possible, slice a small piece (about 1" square) of fat from the outside of the steak.
Light your grill or cooktop, and set up for direct high heat.
Over the direct high heat, place your cast iron skillet or griddle and let it come up to temperature. You'll want it hot, like really hot, so let it preheat.
To the hot pan, add the small piece of the beef fat trimmed from the steak. Let that fat render for 30 seconds or so in order to create a nice oily surface. If you don't have a wedge of beef fat, wagyu tallow also works well.
Place the wagyu steak in the rendered fat. Press down on it with a spatula to ensure the best contact with the hot pan, and resist the urge to move it. Allow the steak to sear for 60-90 seconds.
Flip steak, cover and cook for an additional 60 seconds. Then, check the internal temperature with a reliable quick read thermometer. The temp should be about 115°F. Remove from heat, and place on a warm plate.
Tent the steak with foil, and let it rest for three minutes before slicing. Taste, and add another pinch of salt (if needed) and serve.