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1
Preheat oven to 325 degrees F. Butter a 9 by 12 1/2-inch baking sheet pan, line with parchment paper, and butter the paper.
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2
Finely grate the skin (zest) off all 3 clementines, making sure you peel only the orange skin and not the bitter white pith.
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3
Puree the zest with 1/4 cup of the sugar in a food processor until minced, about 1 to 2 minutes.
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4
Whisk the flour, cornstarch, and salt in a bowl.
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5
In a standing mixer fitted with the paddle attachment, cream the butter with the remaining 1/2 cup sugar on medium, until light and fluffy, about 8 minutes.
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6
Add the clementine-sugar mixture and beat for another minute.
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7
While mixing on low speed, add the dry ingredients and mix just until combined.
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8
Do not overwork the dough.
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9
Transfer dough to prepared pan, spreading it out as evenly as possible.
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10
Cover with plastic wrap, then press and flatten using your hands over the top to smooth and even out the dough.
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11
Cut the dough into bars about 1 by 3 inches with a sharp knife, while still in the pan.
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12
Prick a decorative pattern in the shortbreads with a fork or a wooden skewer.
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13
Bake until golden, about 1 hour and 10 to 20 minutes.
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14
Let stand for 5 minutes and then re-cut the bars along the previous cuts.
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15
Let cool completely on racks.
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16
Store the cookies in a sealed container for up to 2 weeks.
