Heat the oven to 350F and grease an 8x8-inch baking dish. The dish should be ready before the batter is mixed so it can go straight into the oven at the right depth.

Whisk the drained whole-kernel corn, creamed corn, sour cream, melted butter, and egg in a large bowl until evenly combined. The mixture should look glossy, creamy, and loose, with whole corn kernels suspended throughout.

Stir in the Jiffy Corn Muffin Mix just until no dry patches remain. Stop mixing once the batter looks thick and spoonable, because overmixing can make the casserole heavier and more bread-like.

Spread the batter into the prepared dish and bake for 50 to 55 minutes. By about the 40-minute mark, you should notice a sweet cornbread aroma and see the edges turning golden.

Check for doneness, then let the casserole rest for at least 10 minutes before serving. The top should be golden brown, and when you gently tap the side of the dish, the center should show only a slight, stable wobble rather than a loose slosh.