Mix together the starter, water, and flour in a bowl. Cover loosely with plastic wrap and let it sit overnight. (Note: If your starter is very thick, add a bit of extra water so it will mix evenly into the cake mixture.)
In the morning, preheat the oven to 350 degrees and grease a 9 x 13 inch pan.
In a small bowl, mix together the cocoa powder, baking soda, and salt. In a large bowl, stir together the eggs, maple syrup, coconut oil, and vanilla extract. Stir the cocoa powder mixture into the egg mixture. Then add the flour mixture that was sitting overnight, and stir to fully combine. (Again, if the starter mixture seems too thick to blend evenly into the chocolate mixture, add a bit of water to it so it will blend in well.)
Transfer the batter to the prepared pan. Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.