Heat reserved skillet from Pan-Seared Steak, without washing, over high heat. Carefully pour in balsamic vinegar; using a wooden spoon, scrape up any browned bits. Reduce to a syrupy consistency, about 35 seconds. Add salt and any juices that have collected under steak; stir to combine. Remove from heat. Add butter; swirl in pan until incorporated. Pour over steak, and serve immediately.