Preheat the oven to 425°F and spray a doughnut pan with nonstick cooking spray.
In a large bowl, sift together cake flour, cocoa powder, sugar, baking powder, and salt.
Add buttermilk, eggs, oil, and vanilla, mixing gently until combined (the dough will be thick).
Fill each doughnut cup about halfway with the dough (you can use a piping bag for easier filling).
Bake for 4-5 minutes, or until the tops spring back when touched, then cool in the pan for 3 minutes before removing.
Finish with your favorite toppings, like chocolate ganache and sprinkles.
