Begin by preheating your oven to 425°F (220°C).
Place a cast-iron skillet inside the oven to warm up.
In a large mixing bowl, combine cornmeal, flour, baking soda, and salt, whisking them together thoroughly.
In another bowl, mix buttermilk, egg, and melted butter until well blended.
Slowly blend the wet mixture into the dry ingredients, stirring softly until they are just combined.
Carefully take the hot skillet out of the oven, add vegetable oil, and swirl it around to coat the bottom.
Pour the batter into the skillet, ensuring it is spread evenly. Bake for 20 to 25 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Allow the cornbread to cool for 5 minutes before slicing and serving.