Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the cooked chicken (or beef), black beans, corn, diced tomatoes, tomato sauce, taco seasoning, salt, and pepper. Mix well to combine all ingredients.
In a 9x13 inch baking dish, spread a thin layer of the chicken mixture on the bottom. Place a layer of corn tortillas over the mixture, slightly overlapping them.
Spread half of the remaining chicken mixture over the tortillas, followed by a layer of sour cream, and then sprinkle with one cup of cheddar cheese.
Repeat the layering process: add another layer of tortillas, the remaining chicken mixture, another layer of sour cream, and top with the remaining cheddar and Monterey Jack cheeses.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.
Once baked, remove from the oven and let it cool for about 10 minutes. Garnish with chopped cilantro if desired before serving.