Preheat the oven to 200°C/390°F (without fan).
Grease a cast-iron skillet (26 cm/10.5 inch) with butter or non-stick baking spray. (See Kitchen Notes below for using other pans.)
In a large bowl, mix together the cornmeal (or polenta), flour, sugar, salt and baking powder.
In a measuring jug, whisk together the milk and egg.
Pour the milk mixture into the bowl of dry ingredients.
Lightly mix until everything is just combined.
Last, mix through the melted butter.
Pour the mixture into the prepared skillet (or cake pan).
Bake for 20 to 25 minutes, or until the top of the cornbread is lightly golden.
Serve warm, but allow the cornbread to rest for about 5-10 minutes before serving (not crucial, but the cornbread will be very soft if served straight from the oven).