01 - Preheat oven to 350°F. Grease a 9x9-inch baking dish or similar casserole dish with butter or cooking spray.
02 - In a large mixing bowl, add the drained whole kernel corn, creamed corn, sour cream, melted butter, and Jiffy corn muffin mix. Stir with a silicone spatula or wooden spoon until just blended—do not overmix.
03 - If using cheddar cheese, fold it into the batter until evenly distributed.
04 - Pour the mixture into the prepared baking dish and spread the top evenly with the back of a spoon or spatula.
05 - Bake for 45–50 minutes, or until the top is golden brown and the center is set (a toothpick inserted should come out clean).
06 - Remove from the oven and let the casserole rest for 5 minutes before serving to allow it to set.