Remove the skirt steak from the fridge 20–30 minutes before cooking so it comes to room temperature. Pat dry with kitchen paper.
In a small bowl, mix the sea salt, black pepper, garlic powder and onion powder. Press the seasoning into both sides of the steak so it sticks.
Heat a heavy-based frying pan or cast-iron pan over a high heat until it just starts to smoke (about 3–5 minutes). Add the olive oil and swirl to coat the pan.
Add the steak and sear for 2–4 minutes on each side for medium-rare (4–5 minutes for medium). In the last minute, add the butter and spoon it over the steak to baste.
Transfer the steak to a board and leave to rest for 5–10 minutes. Slice thinly against the grain and serve.