Tajin seasoning—a blend of Mexican chiles, lime juice, and sea salt—is available in supermarkets. Or swap in another chili powder.
Make pico de gallo: Combine tomatoes, onion, cilantro, salt, cayenne, 3 tablespoons of the lime juice, and 2 tablespoons of the oil in a bowl.
In a separate bowl, stir together beans, cumin, and remaining 1 tablespoon each lime juice and oil.
Divide half of pico de gallo (about 1/4 cup) among 4 large glasses or pint jars. Top each evenly with romaine, chicken, corn, guacamole, tortilla chips, black bean mixture, and remaining pico de gallo. Top each with 1 tablespoon sour cream and 1 tablespoon pepitas. Sprinkle with Tajín, if desired.