PREPARE: Heat the oven to 180°C / 350°F / Gas 4 and line a 20x30cm (8x12in.) oven dish with baking parchment paper.

CREAM BUTTER AND SUGAR: Weigh the sugar into the bowl of a stand mixer, add the lemon zest, then use your fingers to rub the lemon zest into the sugar releasing all of the fragrant oils. Cube the butter and add to the mixer. Beat butter and sugar together using the paddle attachment until light and fluffy, 3-4 minutes.

MIX DRY INGREDIENTS: In a medium bowl, whisk together the flour, cornstarch and sea salt. Add to the butter mixture and mix on low speed until just combined, scraping down the sides of the bowl to ensure all of the butter is mixed in. Knead gently with your hands until the mixture begins to clump together, then pour into the prepared oven dish.

SMOOTH AND MARK: Use the back of a spoon to smooth the shortbread dough, pressing it down in an even layer. Use a small, sharp knife, mark out the surface of the dough into rectangles, then prick it all over with a fork.

BAKE: Bake in the preheated oven for 30 minutes until a light golden colour. While the shortbread is baking, rub together the remaining sugar and lemon zest for the topping.

SPRINKLE AND COOL: When the shortbread is ready, remove it from the oven and sprinkle immediately with the lemon sugar topping. Allow to cool for 10 minutes in the baking dish before transferring to a cooling rack. Slice into rectangular fingers before serving.