200g plain flour, preferably Italian OO
200g very soft unsalted butter
seeds from 1 vanilla pod
vanilla or ordinary caster sugar for sprinkling
Process again until the soft mixture coheres and begins to form a ball, loosely clumping around the blade. Turn this out on to a Swiss roll tin and press to form an even (or as even as you can make it) layer, using fingers or the back of a spoon, or both. Be patient, it will fit smoothly.
Using the tip of a sharp knife cut the pressed-out shortbread into fingers. I make two incisions lengthways - i.e. to form three layers - and then make 10 cuts down - so that you end up with 11 fingers per layer. Obviously, the aim should be to cut at regular intervals but don't start getting your ruler out. Just go by eye: uniformity is the province of the conveyor belt not of home cooking. Use the tines of a fork to make little holes in each marked-out biscuit: I press down about three times, diagonally, on each finger.
Now that you've pressed, incised, and punctured, slide the Swiss roll tin into the oven and bake for about 20-25 minutes, by which time the shortbread will be pale still, but not doughy. Expect a little goldenness around the edges, but shortbread should be not crisp but melting.
Remove the tin from the oven and let cool for 10 minutes or so, before removing, with a palette knife and your fingers, to a wire rack. Sprinkle with sugar and leave them to cool completely before storing in a tin.