Preheat the oven to 350 °F. Butter and flour a 13" x 9" cake pan or use pan spray. Line the pan with parchment paper long enough so that covers the bottom of the pan and hangs over two sides.
Put 2 oz Dutch process cocoa powder into a small heat-proof bowl. Whisk 4 oz boiling water into the cocoa. Set aside to cool while preparing the other ingredients.
Sift 7 oz cake flour with 1 teaspoon baking powder and ½ teaspoon table salt, set aside.
Combine8 oz vegetable oil,4 large eggs,4 oz whole milk, 1 tablespoon vanilla extract and 16 oz granulated sugar in a mixer bowl. Mix on medium speed to combine the ingredients, continue mixing for 2-3 minutes to emulsify the ingredients. Add 8 oz sourdough discard and mix to combine.
With the mixer running on low, add the flour and then add the cocoa mixture. Mix until completely incorporated.
Pour the batter into the prepared pan. Bake until the top springs back when lightly pressed and a toothpick inserted in the middle of the cake comes out clean, about 30 minutes.
Cool in the pan for 20 minutes. Use the parchment paper to slide the cake out of the pan onto a cooling rack.
Serve with a sprinkle of powdered sugar or your favorite frosting. Cool completely before icing.
Notes
If measuring the flour by volume use the “dip & sweep” method. That is, dip the measuring cup into the flour bin, overfill it, then sweep away the excess.