In a large mixing bowl (not your stand mixer bowl), combine the milk, sourdough starter, and flour. Stir until all the flour has been moistened, but don't worry about getting out every single lump. Cover the bowl with plastic wrap or a dishtowel and allow to sit for 2 hours.
After the 2 hours is up, preheat your oven to 350°F and generously grease a 9 x 13" baking sheet, then line the bottom with parchment paper for easy release.
In your stand mixer or another large bowl, mix the sugar, oil, vanilla, salt, baking soda, and cocoa powder until combined. Add the eggs one at a time, scraping down the bottom and sides of the bowl after each addition.
Add the sourdough mixture to the bowl and beat on low speed until it's totally combined into the batter. It's very sticky, so it does help to stop the mixer and scrape everything down a few times to make sure everything is incorporated.
Pour the batter into your prepared pan and bake for 30-40 minutes. You'll know it's done when the top is very shiny, and only slightly bounces back when pushed with your finger. Run a knife or spatula around the sides and allow to cool completely in the pan before frosting or removing from the pan.