Preheat the Oven: Set your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
Mix the Ingredients: In a large mixing bowl, combine the whole kernel corn, creamed corn, cream cheese, melted butter, and eggs. Stir until smooth and well combined.
Add the Corn Muffin Mix: Gradually add the corn muffin mix, milk, salt, pepper, and garlic powder (if using) into the mixture. Stir until just combined.
Fold in the Cheddar Cheese: Gently mix in ¾ cup of the shredded cheddar, reserving the rest for topping.
Transfer to the Baking Dish: Pour the mixture into the prepared baking dish and spread it out evenly.
Bake: Place the casserole in the oven and bake for 40–45 minutes or until the top is golden brown and the center is set.
Add the Remaining Cheese: Sprinkle the reserved cheddar cheese on top of the casserole during the last 5 minutes of baking to let it melt.
Cool Slightly and Serve: Let the casserole rest for 5–10 minutes before serving.