Combine the milk, sourdough starter, and flour in a large mixing bowl. Cover and let the base rise at room temperature for approximately 2 to 3 hours. Don't look for bubbling to occur as a sign it's ready, as this may not occur. However, after 2-3 hours, the cake base mixture will have expanded, showing it's ready for mixing.
Preheat your oven to 350°F for about ten minutes before placing the cake inside.Lightly coat the 9"x 13" pan- or the Bundt cake pan with shortening or oil. Bakers may choose to use butter instead, which can offer a little added flavor boost. If you plan to remove the cake from the baking pan, you can also choose to flour the pan over the butter. This will help make it easier to remove it and place it on a serving dish. In the medium mixing bowl, mix well the sugar, oil, baking soda, cocoa, vanilla, and salt. Do not expect the mix to be smooth when blended.
Add one egg at a time, blending well after each egg is added. This will smooth out the mixture a bit.
Pour this mixture into the large mixing bowl with the sourdough, milk, and flour base. Gently blend the two together until smooth. This will take several minutes.
Take the batter mix and pour it into the prepared 9” x 13” baking pan or Bundt cake pan.
Bake for 30-40 minutes, checking it regularly after 30 minutes. Actual completion time can vary depending on the efficiency of the oven. You will know it is done when the cake springs back when lightly touched in the center of the cake. Verify the cake is ready with either a cake tester or a fork in the center to make sure it comes out clean with no mixture attached. If you opt to use a Bundt cake pan, you will need to bake it a little longer. Usually, a Bundt pan will addbetween10 and 20 minutes to the baking time. You should begin testing the cake after 35 minutes of baking time every 5-10 minutes.
Remove the cake from the oven once it is done. Do not let it sit in the warm oven after you turn it off as it will overcook it. Place the cake on a cooling rack and let cool for 45 minutes to 1 hour. This will give you plenty of time to make the icing. If you opt to leave the cake in the pan, you can add the icing as soon as it's ready. If you opt to take the cake out of the pan, removal should not occur before the pan is no longer warm to the touch at the bottom.