Prepare the baking dish. Preheat the oven to 350°F. Generously coat a 9 × 13-inch baking dish, or a similarly sized casserole pan, with butter or nonstick cooking spray.
Assemble the corn mixture. Place the melted unsalted butter, corn muffin mix, drained whole-kernel corn, and cream-style corn in a large mixing bowl. Stir thoroughly until the dry mix has been completely incorporated and no dry areas remain.
Add the remaining ingredients. Incorporate the room-temperature sour cream, eggs, and granulated sugar into the corn mixture. Stir until the batter is evenly blended and smooth, with no remaining lumps of dry muffin mix.
Transfer and bake. Pour the completed batter into the prepared baking dish and spread it into an even layer. Bake for 40–45 minutes, checking around the 40-minute mark. The casserole is ready when the edges are golden brown and a toothpick inserted near the center emerges clean or with only a few moist crumbs.
Cool before serving. Remove the casserole from the oven and allow it to rest for 5–10 minutes. This brief cooling period helps the interior firm slightly so portions hold together more easily. Serve warm directly from the baking dish.