Preheat the oven to 350°F. Spray a donut pan well with non-stick cooking spray and set aside.
In a large bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a separate bowl, mix together the buttermilk, sugar, egg, butter, and vanilla until well combined.
Add the wet ingredients to the dry ingredients and mix until just combined.
Spoon the batter into the donut cavities making sure to only fill them about 2/3 - 3/4 of the way full.
Bake for 8-10 minutes or until a toothpick inserted into the donut comes out clean.
Remove from the oven and allow to cool for about 5 minutes in the pan. Remove donuts from the pan and transfer to a wire rack to cool.