Preheat oven to 425°F. Heat oil in a 9-inch cast-iron skillet over medium.
Meanwhile, sift the flour with the sugar, baking powder and salt in a large bowl. Stir in the cornmeal. Add the milk and eggs and stir lightly. Add the melted butter and stir just until blended. Scrape the batter into the hot skillet; the oil should bubble. Transfer the skillet to the oven and bake until the center springs back when gently pressed, about 18 minutes. Turn the corn bread out onto a rack to cool.
