Preheat your oven to 400°F (200°C). Place the cast iron skillet in the oven while it preheats.
In a large bowl, combine the cornmeal, flour, sugar, baking powder, and salt.
In another bowl, whisk together the milk, vegetable oil, and beaten egg.
Pour the wet ingredients into 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.