Heat the oven to 350°F with a rack in the center. Use a small amount of the measured melted butter to grease a 9-inch square baking dish. Drain the whole-kernel corn thoroughly; do not drain the creamed corn.

Whisk the cornmeal, flour, sugar, baking powder, and salt in a medium bowl until evenly combined.

In a large bowl, whisk the egg, sour cream, and remaining melted butter. Fold in both kinds of corn. Add the dry ingredients and stir just until no dry streaks remain. Do not taste raw batter.

Spread the batter in the prepared dish. Bake for 45–55 minutes, until golden at the edges, set in the center rather than liquid, and at least 165°F in the center on an instant-read thermometer. If the top browns early, loosely tent with foil. Continue baking if necessary to reach the endpoint.

Rest for 10 minutes, then divide into eight portions and serve warm. Refrigerate leftovers in shallow containers within two hours, or one hour if above 90°F. Use within 3–4 days and reheat to 165°F.