Preheat the oven to 180°C/350°F/170°FF
Using a stand mixer, beat the butter, sugar and salt on medium for 5 minutes
Add the egg and beat for another 2 minutes
Sift the dry ingredients
In a small jug, combine the milk and vanilla extract
Add the dry ingredients to the mixer in 3 batches, alternating with the milk and vanilla
Melt extra butter and brush it lightly into the cavities of the donut pan
Transfer the donut mixture to a disposable piping bag (or a large snap lock bag will do), then cut the end off the bag to make an opening
Pipe the mixture into 10 donuts, filling the cavities about halfway
Bake for 10 minutes, until lightly golden and cooked through
Remove from oven and rest in the pan for 10 minutes, before turning out to a cake rack to cool