Preheat the oven to 180C/160F. Grease your mini donut mould.
In a medium sized bowl, beat the butter and sugar together until the mixture is very light and fluffy.
Add the egg a little at a time, beating well. Beat in the vanilla extract.
Sift the flour, baking powder and salt together. Using a large metal spoon, fold in half of the flour to the creamed mixture, taking care not to over mix.
Fold in half of the buttermilk. Finally, gently fold in the remaining flour and buttermilk until the mixture is smooth but again taking care not to over mix.
Scrape the mixture into a piping bag fitted with a medium sized round nozzle. Fill the donut moulds no more than half full.
Alternatively, you can just spoon the mixture into the donut mould, but I find this quite fiddly and on my first try I ended up over filling the moulds - if you have a steadier hand than I do you might be ok!
Speaking of overfilling, its pretty important that you don't because the holes will close up and you won't have that classic donut shape.
Place the mould on a baking tray and place in the oven.
Bake for 10-12 minutes or until risen and springy. Remove from the oven and leave to cool for 5-10 minutes before turning out onto a wire rack.
Give the mould a quick wash and grease again before you make another batch.
When the donuts are cold, dip them in the glaze. Place on a wire rack with a piece of parchment underneath to catch any drips.