Make the spice rub: whisk together the kosher salt, ground cumin, sweet paprika, black pepper, and garlic powder in a small bowl and set aside.
Prepare the steaks: pat steaks dry and rub all sides with a neutral high-smoking-point oil, then press the spice rub evenly onto all surfaces.
Rest the steaks: let the rubbed steaks rest at room temperature for 40–60 minutes, or refrigerate uncovered up to 48 hours; if refrigerated, bring to room temperature for 30–60 minutes before cooking.
Make the harissa butter: in a medium bowl, whisk together the room-temperature unsalted butter, harissa paste, 1/8 tsp salt, minced mint, chopped chives, and lemon zest until smooth.
If grilling, form the harissa butter into a log: transfer the butter to plastic wrap, roll into a log, and refrigerate until semi-firm, about 30–45 minutes; soften slightly before serving if too firm.
Prepare the grill: heat the grill to about 600°F with the lid closed (about 10–15 minutes), clean with a wire brush, and oil the grates using tongs and an oiled paper towel.
Grill the steaks: place steaks on the hot grill and cook with the lid open until desired doneness, about 4–5 minutes on the first side and 3–5 minutes on the second side for medium-rare (130°F target; remove about 5°F early).
Skillet-to-oven alternative: preheat oven to 275°F during the steak resting time. Heat a large cast-iron skillet over high heat until smoking, add 1 tbsp vegetable oil, sear steaks 3–4 minutes without moving, flip, add about 3 tbsp of the harissa butter to baste for 45 seconds, then transfer skillet to oven and cook until an instant-read thermometer reads 130°F for medium-rare (about 5–7 minutes).
Rest and serve: transfer steaks to plates, top each with a generous slice of harissa butter, loosely tent with foil, and rest 5–10 minutes before slicing against the grain.