Sous Vide the porterhouse: Sprinkle the porterhouse with the salt and pepper, put it in a large (gallon) vacuum pouch, and vacuum seal. (The vacuum sealed steak can be refrigerated for up to 3 days, or frozen for months.) Set the sous vide to 136°F/58°C for medium. (125°F/51.5°C for rare, 131°F/55°C for medium-rare, 141°F/60.5°C for medium well.) Drop the porterhouse in the sous vide water and cook for 1 1/2 hours to 6 hours. (Cook for at least 2 hours if the steak is frozen.)

Sear and butter baste the porterhouse: While the steak is cooking sous vide, preheat the pan over medium-high heat. (I preheated my cast iron pan in a 425°F oven for at least 20 minutes, then moved it to the stove top over medium-high heat.) Remove the porterhouse from the vacuum bag and pat dry with paper towels. Add the butter to the pan, and the moment it stops foaming, add the porterhouse. Sear the porterhouse, basting with butter, until it is well browned, about 1 minute a side. Remove to a cutting board, carve, and serve.