Remove skirt steak from the fridge 20–30 minutes before cooking to come to room temperature. Pat dry with paper towels.
In a small bowl, mix kosher salt, black pepper, garlic powder, and onion powder. Season both sides of the steak, pressing gently to adhere.
Heat a heavy-bottomed or cast-iron skillet over high heat until smoking hot (3–5 minutes). Add olive oil and swirl to coat.
Place the seasoned skirt steak in the skillet. Sear for 2–4 minutes per side for medium-rare (4–5 minutes per side for medium). In the last minute, add unsalted butter and baste the steak.
Transfer steak to a cutting board and let rest for 5–10 minutes. Slice thinly against the grain and serve.