Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish.
In a large skillet, cook the ground beef and onion over medium heat until the beef is browned and the onion is tender. Drain any excess fat.
Stir in the taco seasoning mix, diced tomatoes with green chilies, and corn. Cook for 2-3 minutes until well combined. Remove from heat.
In a large mixing bowl, combine the hashbrowns, 1 cup of shredded cheddar cheese, cream of chicken soup, sour cream, and milk. Mix well and season with salt and pepper to taste.
Spread half of the hashbrown mixture evenly in the prepared baking dish. Layer the taco meat mixture over the hashbrowns, then top with the remaining hashbrown mixture.
Sprinkle the remaining shredded cheddar cheese and black olives (if using) over the top.
Bake in the preheated oven for 40 minutes, or until the casserole is bubbly and golden brown on top.
Let cool for 5 minutes before serving. Garnish with fresh cilantro, if desired.