Whizz the flour, sugar and cornflour in a food processor to sieve and mix briefly, then add the vanilla and drizzle in the olive oil, pulsing the food processor blades until you get a soft, golden dough. Wrap and chill for 30 mins to rest.
Heat the oven to 180C/160C fan/gas 4, and line a baking sheet with parchment or a baking mat. Roll the dough out on a lightly floured work surface to a 5mm thickness and use a round or fluted cutter, about 6cm diameter, to cut out shortbread rounds. Use a small palette knife to transfer to the baking sheet. Can be frozen on the baking tray, then transfered to a box when solid. Will keep for up to three months.
Sprinkle the 1 tbsp sugar over the biscuits and bake for 15-20 mins until golden brown. Leave to cool for a few mins to firm up on the tray, then transfer to a cooling rack to cool completely. Add 2-4 mins to the cooking time if baking from frozen.