Trim excess fat and silver skin from 2 lb. skirt or flank steak. Cut meat into 3–4 pieces for skirt steak or 2 pieces for flank steak; transfer to a rimmed baking sheet.

Mix 2 Tbsp. Diamond Crystal or 1 Tbsp. plus ½ tsp. Morton kosher salt, 2 Tbsp. light or dark brown sugar, 2 tsp. chipotle chile powder, 2 tsp. freshly ground pepper, 2 tsp. garlic powder, and 2 tsp. ground cumin in a small bowl to combine. Sprinkle rub all over meat, patting to adhere. Chill, uncovered, at least 1 hour and (preferably) up to 4 hours.

Prepare a grill for high heat; oil grate. Mix 2 garlic cloves, finely chopped, ¼ cup finely chopped cilantro, 3 Tbsp. fresh lime juice, 2 Tbsp. extra-virgin olive oil, 1 Tbsp. apple cider vinegar, and remaining ¼ tsp. Diamond Crystal or Morton kosher salt in a small bowl to combine. Set dressing aside.

Rub steak all over with 3 Tbsp. vegetable oil and grill, turning every 2 minutes or so, until medium-rare (an instant-read thermometer inserted into the center should register 130°), 5–9 minutes (skirt steak is generally thinner and will have a shorter cook time; flank steak is thicker and will take longer). Transfer to a cutting board and immediately brush on all sides with reserved dressing. Let rest 10–15 minutes.

If pieces are very wide, cut steak with the grain in half lengthwise. Thinly slice meat against the grain. Arrange on a rimmed platter and drizzle juices from cutting board and any remaining dressing over. Sprinkle with flaky sea salt.