Step 1: Prepare the Marinade: In a medium bowl, whisk together tequila, lime juice, honey, minced jalapeños, minced garlic, olive oil, cumin, chili powder, salt, pepper, and chopped cilantro. Mix well to combine.
Step 2: Marinate the Steak: Place the flank steak in a large, resealable bag or a shallow dish. Pour the marinade over the steak, ensuring it's fully coated. Seal the bag or cover the dish and refrigerate for at least 2 hours, but no more than 4 hours.
Step 3: Prepare Grilled Lime Butter: While the steak marinates, preheat your grill to medium-high heat. Place the halved limes directly on the grill grates, cut-side down, and grill for 3-5 minutes until lightly charred and softened. Remove and let cool slightly.
Step 4: Finish the Butter: Once cool enough to handle, squeeze the juice from the grilled limes into a small bowl. Zest one of the grilled lime halves into the bowl as well. Add softened butter, chopped cilantro, salt, and pepper. Mash with a fork until well combined. Set aside.
Step 5: Preheat Grill for Steak: Remove the steak from the refrigerator 30 minutes before grilling to bring it to room temperature. Preheat your grill to high heat (450-500°F). Lightly oil the grill grates.
Step 6: Grill the Steak: Remove the steak from the marinade, discarding any excess marinade. Place the steak on the hot grill. Grill for 4-6 minutes per side for medium-rare (internal temperature 130-135°F), or longer for desired doneness.
Step 7: Rest and Serve: Transfer the grilled steak to a cutting board and tent loosely with foil. Let it rest for 5-10 minutes. This allows the juices to redistribute, ensuring a tender steak. Slice the steak against the grain into thin strips. Serve immediately, topped generously with the grilled lime butter.