Preheat Oven and Prepare Tin: Preheat your oven to 160ºC (320ºF) fan-forced. Grease a 9-inch x 9-inch (22cm x 22cm) square baking tin with butter. Line it with parchment paper, leaving some overhang to help remove the shortbread later.
Prepare Mixing Equipment: Ready your stand mixer with the paddle attachment or a large mixing bowl with an electric hand beater.
Cream Butter: Beat the butter in the mixing bowl at medium speed for 30 seconds, then scrape the sides.
Add Sugar: Sift in the powdered sugar and continue to beat for 1 minute, remembering to scrape the bowl again.
Add Flour: Gently mix in the flour on low speed until a crumbly dough forms. Avoid overmixing; you can finish bringing the dough together with your hands.
Press into Tin: Evenly press the dough into the lined tin. Flatten and smooth the top using the bottom of a glass. If the glass is smooth, you can roll it across the dough as you would with a rolling pin.
Score Biscuits: Score the dough into 18 rectangles using a long knife or pastry scraper, ensuring you cut all the way through. To do this evenly, measure the length of the dough and divide it into three equal sections, marking two straight lines.
Cut Biscuits: Then rotate the pan and measure the width the other way and divide it into six equal sections, marking five lines. Cut all the way through along these lines. You should have 18 rectangles.
Prick Biscuits: Prick each rectangle several times with a wooden skewer or fork, piercing halfway into the dough.
Bake: Bake for 30-35 minutes until the edges are just lightly golden; the rest should remain pale.
Cool Slightly: Let the shortbread cool in the tin for 5 minutes.
Cut Rectangles Again: Recut along the pre-scored lines and prick again at the same holes.
Cool: Leave the shortbread to cool in the tin for another 10 minutes.
Remove From Tin: Using the parchment overhang, lift the shortbread onto a wire rack to cool completely, keeping it in one piece.
Serve: Once cooled, break along the cut lines into individual biscuits and enjoy!