How does this happen? Here's the science for you.
The sugar particles are coated in the vegetable oil so the moisture can't get at them. It's as simple as that. The use of dextrose instead of sucrose also helps as it is a finer and less sweet sugar. That's why those sugar coated doughnuts don't taste overly sweet.
Now we reach the second problem. This dusting sugar is not widely available in the UK. It can be bought from bakery supplies if they will sell it to you but it comes in 12.5kg bags. Far too much for the home baker to use.
So what is there to do?
The solution is fairly simple. Instead of using the above list of ingredients you can coat the sugar in melted cocoa butter. To give the sugar a delicate flavour of vanilla use some vanilla extract in the melted cocoa butter.
You will need a food processor to make the dusting sugar, but at a push you could do it with your finger tips. It is less messy and more efficient in the food processor.
To make your dusting sugar:
10g cocoa butter, melted.
1 teaspoon vanilla extract
50g icing sugar
If you don't have dextrose and it is fairly easy to get from Amazon. See this link then you can use all icing sugar. It will be sweet.
Melt the cocoa butter. I did this by putting the cocoa butter in a small bowl and placing it in a larger dish that contained boiling water. it didn't seem to like the microwave method of melting chocolate. It will only take a couple of minuted to melt. Add the vanilla to the melted butter.
Put the dextrose and sugar mixture into the bowl of the food processor and switch on. Pour the cocoa butter in through the feeding tube. It will mix into the sugar. If any of the butter collects on the base of the bowl then give it a stir and pulse again. Repeat this until all of the cocoa butter has been absorbed. It will not be pure white as the vanilla will colour it slightly. You won't notice this when it's on the cake.
Your dusting sugar is now ready. Store in an air tight jar until ready to use.