Grease a 23cm / 9inch square cake tin and line it with baking parchment.
Whisk together the vegan butter and the caster sugar with an electric mixer until it is light and fluffy, then whisk in the vanilla extract and a good pinch of salt.
Sift together the plain flour and cornflour (cornstarch) then add them to the butter mixture. Stir with a spatula until it forms a dough, it may seem like it is too much flour but it will combine eventually. Don't knead the dough, just mix it until no dry patches remain.
Tip the dough into the prepared tin and use your hands to pack it down as firmly and as evenly as possible. You can flour your hands if it is sticking.
Use a sharp knife to slice the dough into fingers and prick them several times each with the tines of a fork.
Place the tin in the fridge to chill for at least half an hour, until the dough is firm. Meanwhile preheat the oven to 170°C/150°C fan/325°F/gas mark 3.
Bake the shortbread for 45 minutes to 1 hour (mine takes 1 hour). It should be pale golden and firm to the touch.
Remove from the oven and immediately sprinkle with granulated sugar. Leave the shortbread to cool in the tin for 10 minutes then use a very sharp, pointy knife to re-slice the previous cuts.
Carefully lift the shortbread fingers out of the tin and place them on a wire rack to cool completely. Once they are completely cool, store the shortbread in an airtight container.