Preheat the oven to 350°F (175°C). Grease a donut pan.
In a large bowl, whisk together the dry ingredients: flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In a separate bowl, mix the wet ingredients: buttermilk, vegetable oil, egg, and vanilla extract.
Gradually combine the wet and dry ingredients, mixing until just combined.
Fill each donut cavity about two-thirds full with batter.
Bake the donuts in the preheated oven for 10-12 minutes, or until a toothpick inserted comes out clean.
Allow the donuts to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Once cooled, feel free to glaze or dust with powdered sugar as desired.