1) Preheat the oven to 200C/Gas 6. In a large bowl, whisk together the polenta, flour, baking powder, sugar, and salt.
2) In a medium bowl, whisk together the milk and egg. Pour the milk mixture into the polenta mixture and, using a rubber spatula, lightly mix until a thick batter is formed. Stir in the melted butter until just incorporated.
3) Heat the cast-iron mould in the oven until hot. Carefully remove the pan from the oven and brush cornstick slots with butter. Spoon the batter evenly into the slots. Bake until lightly browned and a toothpick inserted in the centre of a cornstick comes out clean, about 25 minutes. Let cool slightly. Using a small knife, pop each cornstick from its slot and serve warm.