2) Finely grate the zest from all three clementines, making sure you peel only the orange skin and not the bitter white pith. Puree the zest with 50g of the sugar in a food processor until minced, about one to two minutes.
3) Whisk the flour, cornflour and salt in a bowl. In a standing mixer fitted with the paddle attachment, cream the butter with the remaining 100g sugar on medium, until light and fluffy, about eight minutes.
4) Add the clementine-sugar mixture and beat for another minute. While mixing on low speed, add the dry ingredients and mix just until combined. Do not overwork the dough.
5) Transfer the dough to the prepared pan, spreading it out as evenly as possible. Cover with clingfilm, then press and flatten using your hands over the top to smooth and even out the dough. Cut the dough into bars about 2 by 7cm with a sharp knife, while still in the pan. Prick a decorative pattern in the shortbreads with a fork or a wooden skewer. Store in an air-tight container.
6) Bake until golden, about 70 to 80 mins. Leave to stand for five minutes and then re-cut the bars along the previous cuts. Leave to cool completely on racks. Store the cookies in a sealed container for up to two weeks.