Prep oven and pan. Preheat the oven to 350 degrees with the rack in the center. Prepare a 9 x 13 pan with cooking spray.
Mix dry ingredients. In a large mixing bowl, combine the flour, sugar, cocoa, baking powder, baking soda, and salt. Stir until well combined.
Add wet ingredients. Add the vegetable oil, eggs, vanilla, sour cream, and sourdough starter. Stir to form a thick batter. Pour in the cup of boiling water and stir well until a smooth batter forms.
Bake. Pour into prepared baking pan and bake for 35-45 minutes, until a toothpick inserted into the center comes out clean, or with only a few crumbs. (The top may crack.)
Cool. Cover cake with a clean tea towel and allow it to cool directly in the pan. Cool completely before frosting.
Frost and serve. To make the frosting, combine the melted butter and cocoa in a large mixing bowl. Add the powdered sugar and milk, alternating with each other, and beat with an electric mixer until a smooth frosting forms. Frost the cake and serve directly out of the pan.