Preheat your oven to 350 degrees Fahrenheit.
Add the flour, baking powder, baking soda, salt, sugar and cocoa powder to a large bowl and whisk them together until they're combined.
Add the buttermilk to the bowl with the dry ingredients, and then add the egg, canola oil, and vanilla.
Use a wire whisk to combine the wet and dry ingredients just until there are no more streaks of the flour mixture. Scrape down the sides of the bowl and add the batter to a large piping bag.
Grease the donut pan with a little bit of canola oil spray and pipe the batter into the donut pan using a piping bag until each of the cups is only about 1/2 full. Be careful not to overfill as this will cause the donuts to rise too high out of the pan and lose their donut shape.
Bake the donuts at 350 degrees Fahrenheit for about 7-9 minutes until they've puffed up and they spring back to the touch.
You'll likely have to bake these donuts in 2 batches since the recipe makes 24 (that is, unless you're lucky enough to own 2 donut pans!)
While the donuts cool, make the glaze by whisking the cocoa powder and powdered sugar together, adding the milk in a few teaspoons at a time until you achieve the right consistency for dipping.
Once the donuts have cooled completely, dip the tops into the glaze and set the donuts aside so the glaze can dry.