This is a recipe from an old family friend - Joyce Durrant. She and her husband, Maurice, lived two fields away from the farm when I was a child, as they did when my father was young, and we would occasionally traipse over in our wellies to see them. Joyce always had something good to eat, very often this gorgeous, sweet-but-not-too-sweet, buttery, crumbly shortbread. I give imperial measurements as this is how I cook it, but moving to metric is hardly taxing. It can either be baked into thin crisp biscuits, in which case use a swiss roll tin or similar, or (as I prefer) thick stubby chunks, for which use a deeper cake tin. I currently use a 23cm round springform, which is far from ideal as you get funny shaped pieces and risk the middle being under cooked, but since I am reluctant to share with anyone with whom I am not very intimate, I don't tend to worry about it. When I get a proper square cake or brownie tin I might have to start offering these to guests. How unfortunate.
You could use vanilla sugar in place of the caster, if you happen to have some made up.
Preheat the oven to 170°C. Cut the butter into very small dice. Stir into the plain flour with a knife, then rub in with your fingers. This will take a while, but stick with it and you will eventually end up with fine sand. If you fear you are making the butter oily due to very warm hands (something with which I am not blessed) rinse them under cold water for a moment or two. Stir the cornflour into the flour-butter mix and then 4oz of the sugar and the salt. Pour into your chosen tin, spread evenly and pat down. Prick in a few places with the tines of a fork then place in the oven. Bake for 30 minutes (less if you are using a large shallow tin), or until the top is an even, pale gold. Sprinkle the reserved sugar and bake for another 5 minutes before removing from the oven. Cut into individual biscuits immediately, but leave in the tin on a wire rack to cool. Once cool enough to handle easily, they can be removed from the tin and separated.