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 medium bowl, mix together the cocoa powder, baking soda, and salt. In a large mixing bowl, stir together the eggs, maple syrup, coconut oil, and vanilla extract. Stir the dry ingredients into the wet ingredients (egg mixture). Then add the sourdough 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 before adding it to the chocolate mixture so it will blend in smoothly.)
Pour the cake batter into the prepared pan. Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean. Let it cool on a cooling rack before cutting.