How to Make It
Heat oven to 325° F. Butter a 9-inch springform pan and line with parchment; butter the parchment. Whisk together the flour, cornstarch, and salt in a large bowl; set aside.
In a separate bowl, beat the butter, confectioners’ sugar and vanilla with an electric mixer on medium-high until light and fluffy, 2 to 3 minutes. Add the espresso and beat to combine. Reduce the mixer speed to low and gradually add the flour mixture, mixing just until combined (do not overmix).
With floured fingers, press the dough into the prepared pan and prick all over with a fork. Sprinkle with 2 tablespoons of the granulated sugar.
Bake until golden and firm around the edges, 35 to 40 minutes. Cool for 15 minutes in the pan then sprinkle with the remaining 2 tablespoons of granulated sugar. Tilt the pan so that the sugar evenly coats the shortbread, then tip out the excess sugar. Remove the shortbread from the pan and, using a serrated knife, cut into 12 wedges. Transfer to a wire rack to cool completely.
Storage suggestion: Keep the shortbread at room temperature in an airtight container for up to 2 weeks.