Preheat oven to 160°C and grease your donut pan with flavourless oil OR turn on your donut maker (you won't need any oil if you use the latter).
Cream together the butter and sugar in a medium bowl until fluffy. Add in the egg, milk, and vanilla and mix until just combined. The mixture may look curdled but the dry ingredients will pull it together.
Add in the baking powder, flour and cocoa powder and stir until just combined.
Transfer mixture to a medium zip lock bag and cut the corner tip cut off. Pipe the mixture into the donut pans until each is approximately ⅔ full.
If using a donut pan, bake in the oven for around 10 minutes until they spring back when pressed. If using a mini donut maker, bake for 2-3 minutes.
Allow donuts to cool completely before decorating.