Preheat oven to 350°F Coat an 8-inch-square baking pan with cooking spray.
Whisk cornmeal, flour, sugar, baking powder, baking soda and salt in a large bowl. Whisk egg, buttermilk and oil in a separate bowl. Add to the dry ingredients and stir until just combined. Scrape the batter into the prepared pan, spreading evenly.
Bake the cornbread until the top springs back when touched lightly, 25 to 30 minutes. Let cool in the pan on a wire rack for at least 5 minutes. Serve warm or at room temperature.