Preheat your oven to 425°F (220°C). Place your cast iron skillet in the oven to heat up.
In a large bowl, mix together the cornmeal, flour, sugar, baking powder, and salt.
In another bowl, whisk together the milk, vegetable oil, and beaten egg.
Combine the wet ingredients with the dry ingredients and stir until just combined.
Carefully remove the hot skillet from the oven and grease it with a bit of oil or butter.
Pour the batter into the hot skillet and spread it out evenly.
Bake for 20-25 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Let the cornbread cool in the skillet for a few minutes before slicing and serving.