Combine starter, milk, and flour in a large bowl. Cover and let rest at room temp for 2-3 hours or overnight in the fridge.
Preheat oven to 350F. Grease two 9in cake pans.
In the bowl with the starter, mix in the sugar, oil, vanilla, salt, baking soda, cocoa, and espresso powder, if using. Mix in the eggs, one at a time, until completely combined.
Pour the batter into the prepared pans and bake for 30 - 40 min until a toothpick comes out clean.
Allow to cool in pan for 5 - 10 min, then turn out onto a cooling rack to cool completely.
Notes:
To ensure even size cakes, weigh the pans while adding batter.