In a large bowl, cream butter, powdered sugar, and zest until well combined and fluffy.
Add flour, cornstarch, and salt; mix until just combined.
Add fresh squeezed citrus juice and mix until just combined.
Butter a 9-inch tart pan. Divide the shortbread dough into small pieces and distribute it throughout the tart pan. Use your fingers and palm to press the dough into the pan. Try to make it as even as you can on the bottom of the pan and press the dough up into the sides, all the way to the top of the pan. It will take a few minutes to distribute the dough and even it out.
Place a sheet of parchment paper over the dough and fill with pie weights or beans. Freeze the dough for at least 20 minutes.
Once dough is thoroughly chilled, preheat oven to 375° F. Bake tart dough until golden around the edges and mostly baked through, about 22-27 minutes. Remove parchment paper and pie weights, decrease oven temperature to 325° F, and bake an additional 5-8 minutes, until the bottom crust is fully baked through.
Allow to cool completely before filling.