1. Preheat the oven to 350°F (175°C) and grease a donut pan.
2. In a bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
3. In a separate bowl, whisk together buttermilk, vegetable oil, egg, and vanilla extract.
4. Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
5. Add hot water to the batter and mix until smooth. The batter will be thin.
6. Pour or pipe the batter into the prepared donut pan, filling each cavity about 2/3 full.
7. Bake for 12-15 minutes or until a toothpick inserted comes out clean.
8. Allow the chocolate cake donuts to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
Prep Time: 15 minutes | Baking Time: 15 minutes | Total Time: 30 minutes