Prepare the dough:
Preheat the oven to 160C (140C fan forced) / 350F. 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.
Remove from the oven and let cool for 5 minutes before using a knife to cut through the slices (don’t separate them yet).

Cool and serve:
Leave the shortbread to cool completely in the pan until firm, then gently transfer to a wire rack to finish cooling.