Season the steak with salt and pepper on both sides.
Heat a 12-inch well-seasoned cast-iron skillet over medium-high heat until it starts smoking, for about 3 minutes. Add the steak and press on it with a wide spatula to ensure full contact with the pan.
For medium-rare (135°F), cook the steak for 3 minutes per side plus 1 minute on the fatty edge. If the skillet seems to be getting too hot while you cook, lower the heat to medium. For medium doneness (145°F - the temperature recommended by the USDA), add 30 more seconds per side. Please read the Notes section below for more information.
Remove the skillet from the heat and cover it with foil. Let the steak rest and finish cooking in the skillet’s residual heat for 3 minutes.
Check with a food thermometer to make sure the steak is done to your liking. If not, cover it and keep it in the warm skillet for a few more minutes.
Transfer the steak to a cutting board and let it rest there for three more minutes. Top it with butter, slice, and serve.
Video
Notes
If you have a smoke alarm near your kitchen, run your range hood fan at top speed and crack a kitchen window open.
Remove the steak and butter to the counter an hour before cooking to bring them to room temperature.
It's important to use top sirloin, which is a relatively tender cut, and not bottom sirloin, which is tougher and benefits from slow cooking.
I try to write exact recipes, but in a recipe like this, there are quite a few variables, including the steak's thickness and temperature, the type of skillet used, and the stove used (for example, electric stoves tend to get significantly hotter than gas ones). So, please view the recipe as a guideline and know there's a good chance you will need to adjust it to your circumstances.
The best skillet for cooking steaks is a well-seasoned cast-iron skillet. There's no need to add fat to it if it's well-seasoned.
If you don't have a cast-iron skillet, carbon steel and stainless steel skillets are good options, though not as good as cast-iron. If you don't have those, use what you have, but adjust your expectations - you will only get a good sear on the steak with the right tool. Don't use a nonstick pan - you can't get those hot enough to sear a steak.
If you're not using a pre-seasoned cast-iron skillet, add a tablespoon of avocado oil or clarified butter. Heat the oil over medium-high heat until shimmering - for about 3 minutes - before adding the steak.
You can keep the leftovers in an airtight container in the fridge for 3-4 days. Reheat them VERY gently, covered, in the microwave at 50% power. Or better yet, enjoy them cold. They make a great addition to any salad and can replace the chicken in this Cobb salad.