Prepare sous vide water bath for 125°F.
Season steak liberally with salt.
Place steak in sealable bag, putting 3 tablespoons of compound butter on either side of the steak. Seal bag.
Submerge sealed bag into sous vide bath and allow water temperature to return to the selected 125°F. Make sure bags stay submerged.
Allow steak to sit in water bath for 3 hours.
Just before steak is done in the sous vide, prepare your cast iron skillet and heat it up to 450°F-500°F.
Remove steak from water bath and pat dry with a paper towel. Slice a small piece of fat from the steak and put it on the hot cast iron surface, allowing it to render.
Place steak directly onto the rendered fat in the hot cast iron surface and sear for 90 seconds, on all four sides.
Remove steak from pan, let rest for 5-10 minutes.
Slice cross-ways and serve.