Preheat the oven to 200C/400F/Gas 6. Grease a doughnut pan, and set aside.
Whisk together the flour, baking powder, and salt. Set aside.
Whisk together the egg and sugar until combined. Add the buttermilk, oil, and vanilla, and whisk until combined.
Add the dry ingredients, and mix until just combined. Do not overmix.
Spoon the batter into the prepared doughnut pan, filling almost to the top. Bake for 8 - 10 minutes or until a toothpick inserted comes out clean.
Allow to cool for 5 - 10 minutes before transferring them to a wire rack to cool completely.
For the Glaze
Whisk together the icing sugar, cocoa powder, vanilla, and milk until smooth and combined.
Dip each doughnut into the glaze, and leave to set on the wire rack.
Video
Notes
*To make your own buttermilk, simply add 1/2 tablespoon of fresh lemon juice or white vinegar to 1/2 cup (120ml) of milk. Stir to combine, and allow to sit for 5 - 10 minutes until slightly thickened and curdled.Leftover doughnuts can be stored in an airtight container at room temperature or in the fridge for up to 2 days. They also freeze well for up to 2 months. Thaw overnight in the fridge before serving.