Take the steaks out of the fridge 30-60 minutes prior to cooking them so they come up to room temperature.
Preheat your oven to 400F and move the rack to the middle position.
Pat the steaks dry with paper towel and season them generously (both sides) with salt & pepper.
Add the butter to a heavy oven-safe skillet over medium-high to high heat. The pan needs to get HOT, but be sure not to let the butter burn, so once it melts, stir it every 20-30 seconds until the pan is heated (about 4-5 minutes with cast iron over medium-high heat on my gas stove. Gas runs hotter than electric).
Place the steaks in the pan and then continuously spoon the butter over each steak, ensuring that you're keeping the butter moving around the pan so it doesn't burn. Cook the first side for 2 minutes, and then flip (tongs make it easy) and cook the second side for another 2 minutes. Keep spooning the butter over the steaks for this entire time.
Transfer the skillet to the oven and cook until the steaks reach 125-130F (this will yield medium steaks once they're done resting). For medium-rare, take the skillet out of the oven at 120F. A 2" thick steak will need about 10 minutes in the oven for medium doneness, but oven time will vary depending on the thickness of the steak and other factors. For best results, use an instant read thermometer with an external probe as it's very easy to overcook steak. If you have one with an external probe, insert it into one of the steaks before transferring the skillet to the oven. See blog post for more tips if you're unfamiliar with this tool.
After you take the skillet out of the oven, let the steaks rest in the skillet for 5 minutes before cutting into them so they finish cooking and the juices redistribute (very important so the steaks will remain juicy and tender). The leftover butter can be spooned over the steaks and mashed potatoes or whatever you're serving them with.