Prepare the dough: Preheat the oven to 160°C (140°C fan forced) / 350°F. Line a 9-inch square baking pan with baking paper. In a large bowl (or the bowl of a stand mixer), add the sugar and lemon zest, blending together with your fingertips until it resembles wet sand. Beat in the softened butter until the mixture is light and fluffy.​

Incorporate the dry ingredients: Sift in the plain flour and cornflour, gently mixing on low speed until just incorporated. If the dough is too soft, chill it for 10-15 minutes before continuing.​

Shape the dough: Press the dough into the prepared pan, creating an even layer with a flat top. Use a paring knife to slice through the dough into 9 rows. Turn the pan 90 degrees and slice 3 more rows, creating 27 logs. Use a fork to prick the tops 3-4 times.​

Bake the shortbread: Sprinkle the extra caster sugar over the top, then bake for 35 minutes, or until the shortbread just begins to turn golden on top.​

Cool and serve: Remove from the oven and let cool for 5 minutes before using a knife to cut through the slices (don’t separate them yet). Leave the shortbread to cool completely in the pan until firm, then gently transfer to a wire rack to finish cooling.