Bring your steak to room temperature and pat steaks dry with paper towels. Any excess moisture outside of the steaks will prevent a perfect sear.
Season steaks liberally with salt and black pepper right before cooking or if time permits, salt the steaks 40 minutes to 24 hours before cooking (more about why in the post) and season with black pepper right before cooking.
Heat a cast iron skillet or heavy stainless steel skillet over medium-high heat. Add the oil and swirl the skillet to coat the bottom. When the oil is hot, add the steaks to the skillet.
Sear the steaks on the first side until a nice, deep golden brown crust has formed, about 3 to 4 minutes. Using kitchen tongs, flip the steaks and cook for about 3 additional minutes. If your steak has a fatty layer around the edges, using kitchen tongs, turn each steak on its sides to sear the edges (this should take about 1 to 2 minutes).
Reduce the heat to medium-low and add the butter, rosemary, and garlic. After about 30-45 seconds, tilt the skillet so the butter pools to one side of the skillet. Spoon the butter over the steaks to baste them, and continue cooking and basting until the steaks are about 5-10 degrees away from the desired doneness. (The steaks' temperature will continue to rise 5-10 degrees while they rest).
Remove from the heat and transfer to a cutting board or plate and loosely tent with aluminum foil. Let the steaks rest for 10 minutes before serving.
Serve topped with garlic herb steak butter optional), or spoon some of the basting flavored butter from the skillet over the steaks.