Preheat your oven to 350°F (175°C). Starting with a properly heated oven ensures even cooking. While it heats, gather all your ingredients so the process goes smoothly.
Place the softened butter into your casserole dish and set it in the oven briefly until melted. This step allows the butter to coat the bottom of the dish evenly. It also makes mixing easier once you add the remaining ingredients.
Remove the casserole dish from the oven once the butter is melted. Add the drained whole kernel corn, creamed corn, cornbread mix, and sour cream directly into the dish. Stir everything together gently until just combined and smooth.
Place the dish back into the oven and bake for about 30 minutes. The top should become lightly golden brown. The center should be set but still soft and moist when tested with a toothpick.