Mix all the rub ingredients in a small bowl.

Grill the steak

Half an hour before grilling, coat the steak with the oil and pat on all of the rub, coating evenly. Heat a gas grill to medium high (you should be able to hold your hand 2 inches above the grate for 3 to 4 seconds) or prepare a medium-hot charcoal fire. Sprinkle both sides of the steak with salt. If your grill has a hot spot, position the thicker end of the flank steak nearer the hottest part of the fire. Grill until medium rare, 12 to 15 min., turning the steak every 3 to 4 min. to ensure even cooking. The thickest part of the steak will register 135°F to 140°F on an instant-read thermometer.

Transfer the steak to a cutting board and let it rest for 3 to 5 min. Slice across the grain and portion among dinner plates. Immediately cut the chilled butter into 1/4-inch slices and set a slice or two on each serving while the steak is warm, using a scant tablespoon butter per serving.

Drink Suggestions

Beer is the beverage best suited to the variety of strong, smoky flavors here. A pale ale would be great.