Preheat the oven to 400°F (200°C) and grease an 8-inch (20cm) square tin with butter.
Start by making the buttermilk. Mix the milk and apple cider vinegar and set aside to sour for 10 minutes.
Add the cornmeal, flour, sugar, baking powder, baking soda, and salt to a large mixing bowl. Whisk to combine and make a well in the centre. Set aside.
In another bowl, whisk the egg until frothy. Add the starter, melted butter, and buttermilk. Whisk to combine.
Add the milk mixture to the flour mixture and whisk until just mixed. Don't over mix.
Pour the batter into the prepared baking tin. It is quite liquid, like a thin cake batter.
Bake for 25-27 minutes, or until the top is golden and the edges have started to pull away from the sides of the tin.
Cool for 10 minutes in the tin before removing and cooling fully on a wire rack. Store in a sealed container at room temperature for up to 3 days or freeze. Sourdough cornbread is best fresh.