For the rub:
2 1/2 Tbsp. ground cumin
1 Tbsp. chili powder
1 Tbsp. ground coriander
1 1/2 tsp. freshly ground black pepper
1/2 tsp. ground cinnamon
1/2 tsp. dried oregano
Make the rub by mixing all of the rub ingredients in a small bowl.
For the steak:
1 1/2 to 2 lb. flank steak, trimmed of any excess fat and membrane
1 tsp. olive oil
1 tsp. kosher slat
Chipotle Butter
Grilling 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 minutes, turning the steak every 3 to 4 minutes to ensure even cooking. The thickest part of the steak will register 135 degrees F to 140 degrees F on an instant-read thermometer.
Transfer the steak to a cutting board and let it rest for 3 to 5 minutes. 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.
Chipotle Butter
1/4 lb. (1/2 cup) unsalted butter, completely softened at room temperature
2 canned chipotle chiles in adobo, stemmed, seeded and minced
2 Tbsp. fresh lime juice
Scant tsp. ground cumin
1/2 tsp. kosher salt; more to taste
In a small bowl, cream the butter with a wooden spoon until smooth. Add the chipotles, lime juice, cumin, and salt, blending until evenly incorporated. Taste and add more salt if needed. Using plastic wrap to shape and smooth, mold the butter into a log about 1 1/2 inches in diameter. Wrap will in plastic and transfer to the refrigerator (or freezer, if you're in a hurry) to firm.