To prepare steak & rub: Place steak in a large glass dish. Combine oil, garlic, chili powder, cocoa, cumin, cinnamon, pepper, and 1/4 tsp. salt in a small bowl. Rub the mixture on both sides of the steak. Cover and refrigerate for 1 1/2 to 2 hours.
Meanwhile, prepare dressing: Whisk cilantro, yogurt, mayonnaise, lime zest, lime juice, garlic, honey, and salt in a separate small bowl.
To cook steak: Preheat grill to medium-high. Clean and oil the grates (see Tip). Grill the steak to desired doneness, 4 to 5 minutes per side for medium-rare (130-135 degrees F on an instant-read thermometer). Transfer to a clean cutting board and let rest for 4 to 5 minutes before slicing.
To prepare salad: Combine lettuce, corn, beans, bell pepper, and onion in a large bowl. Add the dressing and toss to coat.
Slice the steak thinly across the grain. Divide the salad among 4 plates, top with the steak, and sprinkle with feta, cilantro, and pepitas, if using.
To make ahead: Refrigerate dressing (Step 2) for up to 2 days.
Tip: To oil a hot grill grate, soak a paper towel with vegetable oil, hold it with tongs and rub it over the grate. (Do not use cooking spray on a hot grill.)