Preheat oven to 350°F. Spray a 9x13-inch baking dish with nonstick spray and set aside.
In a large bowl, combine the cake flour, cocoa, flour, baking powder, baking soda, and salt. Set aside.
1½ cups cake flour, ⅔ cup unsweetened cocoa powder, ⅓ cup all-purpose flour, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon kosher salt
In a separate medium bowl, stir the sugar, vegetable oil, and brown sugar together to combine.
1 cup granulated sugar, ¾ cup vegetable oil, ⅓ cup brown sugar
Add in the eggs and mix until just incorporated. Add the buttermilk and mix, then add the sour cream and mix to combine.
3 large eggs, 1 egg yolk, ½ cup sour cream, ½ cup buttermilk
Add in the vanilla and instant espresso, if desired, and mix until incorporated.
1 teaspoon pure vanilla extract, 1 teaspoon instant espresso
Mix the wet ingredients to the flour mixture and mix until just incorporated.
Transfer the cake batter into the prepared baking dish and place into the preheated oven on the middle rack. Bake for 48 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Once baked, remove the cake from the oven and place on a wire cooling rack.
While the cake is warm, poke holes into the cake using the handle of a wooden spoon or a clean dowel rod. Set aside while you make the milk mixture.