Preheat the oven to 325 degrees.
On a 9x13 inch baking sheet, line it with parchment paper; butter the paper.
Finely grate the skin (zest) off all 3 tangerines, making sure you peel only the orange skin and not the bitter white pith.
Puree the zest with 1/4 cup of the sugar in a food processor until minced, about 1 to 2 minutes.
Whisk the flour, cornstarch, and salt in a bowl.
In a stand 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.
Add the tangerine zest & 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.
Transfer the dough to the prepared pan, spreading it out as evenly as possible.
Cover it with plastic wrap, then press and flatten it using your hands over the top to smooth and even out the dough; remove the wrap.
Cut the dough into bars about 1 by 3 inches with a sharp knife or a pizza cutter, while it's still in the pan.
Prick a decorative pattern in the shortbreads with a fork or a skewer.
Bake them until they're golden, about 1 hour and 10-20 minutes.
Let them stand for 5 minutes and then re-cut the bars along the previous cuts.
Let them cool completely on racks.
Store the cookies in a sealed container for up to 2 weeks.