Top Butt Steak with Whiskey Mustard Sauce

Pat steak dry and sprinkle with 1 teaspoon salt and 3/4 teaspoon pepper. Heat oil in a 12-inch heavy skillet over medium-high heat until it shimmers, then sear steak, turning once, about 8 minutes total for medium-rare. Let steak rest on a platter, uncovered, 10 minutes. While steak rests, add shallot to skillet and cook, stirring, until softened, about 2 minutes. Add whiskey and carefully ignite with a long kitchen match (use caution; flames will shoot up). When flames die down, whisk in broth and mustard and boil until sauce is reduced to about 1/2 cup, about 6 minutes. Add meat juices from platter, then whisk in butter and vinegar. Slice steak and serve with sauce.