In a large bowl, mix the flour, cornmeal, starter, and water (or water kefir). Let mixture sit for at least an hour and up to 12 hours.
Preheat oven to 350 degrees. Put a buttered cast iron skillet into the oven. If you don't have one, just butter a square Pyrex (or similar pan), but do not put it into the oven yet.
Add eggs, softened butter, and maple syrup and mix.
Add baking powder, baking soda, and salt and mix.
Immediately put into your cast iron pan ( I use a 10 inch) and put the pan in the oven.
Bake for 35-45 minutes or until a toothpick inserted comes out clean.