Preheat oven to 400°F and grease a 9" ovenproof skillet with cooking spray. In a medium bowl, stir together corn muffin mix, sour cream, egg, corn, and melted butter. Pour into prepared skillet and bake until golden, 15 minutes.
Meanwhile, in a large skillet over medium heat, heat oil. Add onion and cook until soft, 5 minutes, then add garlic and cook until fragrant, 1 minute more. Add taco seasoning and season with salt. Add chicken and ⅔ cup enchilada sauce and stir until combined.
Poke holes all over cornbread with the bottom of a wooden spoon. Pour remaining ⅓ cup enchilada sauce over poked holes and top with chicken mixture.
Top with both cheeses and bake 20 minutes more.
Garnish with cilantro and sour cream before serving.
