1. Grease two loaf pans with the butter, making sure to get all the way up the sides and in the corners. Sprinkle the cornmeal into the pan and rotate and tap the pan around to coat all the butter with cornmeal. Reserve the remaining cornmeal. Set aside.
2. In a large bowl, combine your active sourdough starter with only 2 cups of the flour and water (reserve the remaining cup of flour for the morning). What you have created is called a poolish or levain. Cover the bowl with plastic wrap and allow it to sit on the counter overnight (or 7-10 hours).