Heat the oven to 160°c and line a baking sheet with parchment paper.
Stir the sugar, vanilla, and salt into the flour. Rub in the butter with your fingertips (you need a light touch with shortbread) and then squeeze gently to form a crumbly dough.
Roll out the dough between two sheets of parchment paper and then sprinkle with the crunchy sugar. Roll o