Pre heat your oven to 180C / 350F / Gas mark 4 and prep your doughnut tin by lightly greasing it. Prep all your ingredients so you have them to hand for the vanilla seeds, cut your vanilla pod in half and store one half of it away again and with the other half with a sharp knife split it length ways then with the back of the knife scrap along the inside to harvest the seeds.
In a medium sized bowl whisk the vanilla sugar, egg and vanilla extract together until pale.
Then add in the natural yogurt, buttermilk, apple sauce and vanilla seeds from your vanilla pod and give it all a good mixing until everything is well combined.
Gently fold in the flour, baking powder and salt to the wet ingredients.
Spoon or pipe the mixture into your donut pan and pop it into the oven for 12 to 15 minutes. Check on them after 12 minutes, they may need a few more minutes if they still feel a little soft. But be careful as these are very pale once baked.
Once baked take them out of the oven and allow them to sit in the pan for a few minutes before removing them and letting them fully cool down on a wire rack.
Once they are fully cooled down in a small bowl place the icing sugar and the drop of vanilla extract and drop of water to it and mix till you have the right consistency. You want it pourable but thick enough to not completely run off. It should stick to the back of your spoon. Add a tiny drop more water if you feel it is still too thick.
Spread it over the top of each donut then cover with sprinkles liberally then enjoy.
Baked donuts don't keep for very long, thats why I always make small batches, but kept in an airtight tin at room temperature they should last for 3 days.