1Combine the flour, cornstarch, and salt in a medium bowl. In a stand mixer fitted with the paddle attachment, beat the butter, sugar and vanilla on medium speed until smooth and creamy, 2 to 3 minutes. Add the flour mixture and mix on low speed until the dough begins to form moist clumps, about 1 minute.
2Turn the dough out into the prepared pan. Using lightly floured fingertips, press the dough into the pan to form an even layer.
3Freeze the dough while the oven heats.
4Position a rack in the center of the oven and heat the oven to 325F.
5Bake the dough, rotating the pan halfway through, until the top looks dry and golden brown, 30 to 40 minutes.
1Whisk lemon juice, a pinch of kosher salt, and remaining 1 1/2 cups powdered sugar in a small bowl until smooth.
1Spread a thin layer of icing over the top of the shortbread cake with a small offset spatula, leaving a thin border of crust exposed. Add berries on top.