Crumble cornbread into fine crumbs; set aside. Stir together the mayonnaise, chili powder, salt, and chopped cilantro, if desired. In a 4-quart glass bowl or trifle dish, place half of the lettuce, then sprinkle with half of the cornbread crumbs. Spread half of the mayonnaise mixture on the cornbread layer. Layer half each of the corn, black beans, tomatoes, onions, and cheese. Repeat layers with remaining ingredients. Refrigerate for 2 hours before serving.