In a large bowl mix together the starter milk, cornmeal and flour.
Cover and let sit on the counter for 8-24 hours.
Preheat oven to 350F.
In a small bowl, whisk together the maple syrup, eggs, melted butter, and sea salt.
Stir the wet ingredients into the soured batter.
Sprinkle the baking powder and baking soda over top and stir well to incorporate.
Pour the batter into a greased 8″ cast iron skillet (or 12 muffin pans, or 2 5″x9″ bread pans).
Bake 35 minutes (or 15 for muffins).
When golden brown and a toothpick inserted into the middle comes out clean, remove from oven and allow to cool for a few minutes before slicing.