Preheat the oven to 350 degrees F. Grease a Bundt pan well with butter. Make sure to get into all the creases.
In the bowl of a stand mixer fitted with the paddle attachment, add the oil, sugar, eggs, sourdough discard, vanilla, coffee, and milk. Mix well.
In a separate medium bowl, add the flour, cocoa powder, salt, and baking soda. Mix well to combine.
Add the dry ingredients to the wet ingredients. Mix until all the ingredients are combined. Do not overmix, but make sure there are no lumps of sourdough starter in the batter.
Pour the chocolate cake batter into the prepared Bundt pan.
Bake for 40-45 minutes or until a toothpick is inserted and comes out clean. Allow the cake to cool on a wire cooling rack and then turn out the cake onto a plate and let it cool completely.