Heat the oven to 350°F (175°C) and lightly grease a standard donut pan. In a medium bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.

In a second bowl, whisk the room-temperature egg, buttermilk, vegetable oil, and vanilla extract until blended. Pour the wet mixture into the dry ingredients and stir only until combined, leaving a few small lumps if necessary.

Transfer the batter to a piping bag or a zip-top bag with one corner snipped off. Pipe the batter into the donut cavities, filling each one about two-thirds to three-quarters full while keeping the batter away from the center posts.

Bake for 10–12 minutes, or until the donuts spring back lightly when touched and a toothpick inserted near the center comes out with only a few moist crumbs. Do not wait for the tester to look completely dry, since overbaking can make the donuts dry.

Cool the donuts in the pan for about 5 minutes, then transfer them to a wire rack. Let them cool completely before glazing so the chocolate topping stays thick and glossy.

For the glaze, whisk the sifted confectioners' sugar, cocoa powder, milk or cream, vanilla, and salt until smooth. If the mixture is too thick for dipping, add more liquid one teaspoon at a time.

Dip the tops of the cooled donuts into the glaze and allow the excess to drip back into the bowl. Return the donuts to the rack and add sprinkles, chopped nuts, or flaky salt before the glaze sets.