Position a rack in the center of the oven and heat the oven to 325°F.
Lightly butter a 9 by 13 inch baking pan.
Dissolve the instant espresso in the vanilla. Set aside.
In a medium bowl combine the flour, cornstarch, cardamom and salt. Whisk and set aside.
In the bowl of your stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth, about 2 minutes. Add the confectioners' sugar, the espresso and vanilla mixture and beat until creamy.
Add the flour mixture and beat until well combined, stopping to scrape the bottom and sides of the bowl once or twice.
Transfer the dough into the pan and spread it pressing lightly to ensure the dough is evenly distributed into the pan.
Before baking, evenly cut the dough (cutting all the way-through) into 40 cookie bars. Prick each cookie with a fork 3 times.
Bake, turning the pan once after 15 minutes. Continue baking until the top is dry and lightly golden, it should take about 15 to 20 minutes more.
Remove the shortbread from the oven and cool in the pan on a wire rack for a few minutes before using a small knife to re-cut the cookies along the lines. Let the cookies cool entirely before removing them from the pan.
Shortbread can be stored at room temperature for up to two weeks. Cookies can also be frozen for up to six months. Pack shortbread in an airtight freezer container with parchment or wax paper between layers of cookies.