Sift together the icing sugar and salt into a medium bowl.
In the bowl of a stand mixer with a paddle attachment, cream the butter on low speed with the sifted icing sugar for about 2 minutes until the butter is pale.
Add in the egg and vanilla extract, beat on medium speed for 2 minutes and scrape down the sides of the bowl. It will look separated, but it will come together once you add the flour.
When creaming the butter with the sugar and eggs, avoid incorporating too much air because the dough will crumble easily when you roll it out. The goal is to make a flexible and pliable dough to form into your tart pans.
Mix in the flour on low speed for 30 seconds or so until there is no more dry flour and dough starts forming. Avoid over-kneading the dough. It will yield a tough pastry and it will shrink more when baked.
Transfer onto a clean work surface, gather and form into a disk, wrap it in plastic and let it chill in the fridge for at least 30 minutes to firm up. You can also make this in advance for up to 3 days.
Tips/notes:Dough scraps can be either re-rolled into more tarts or made into cookies.