In a bowl, mix flour, cornmeal, sugar, baking powder, baking soda, and salt. Mix well then set aside.
In another bowl, mix egg, sour cream, milk, and melted butter. Mix well.
After mixing the egg mixture, pour it into the flour mixture. Mix together to combine.
Brush the rectangular baking pan with butter then pour the cornbread mixture into the baking pan carefully.
Preheat the oven to 350 degrees F.
Then bake the cornbread mixture for 25 minutes.
After 25 minutes, check the inner part of the cornbread using a toothpick. When the toothpick comes out clean it means the cornbread is cook already.
Let it cool for 10 minutes before slicing. Serve and enjoy!