In a medium bowl, whisk together the flour, cornstarch, and salt until well blended.
In a stand mixer set on medium speed, beat the butter until creamy. Beat in the granulated sugar until light and fluffy, about 2 minutes, scraping down the sides and bottom of the bowl as necessary. On low speed, add the flour mixture in two or three additions until it forms a soft dough. Wrap the dough with plastic wrap and chill for 30 minutes.
Preheat the oven to 350°F Line a baking sheet with parchment paper or leave ungreased.
On a lightly floured surface, roll the dough out to a ¾-inch-thick slab, about 5 by 8 inches. (I use a kitchen ruler to measure and press the edges into shape.) Transfer to the baking sheet and bake until lightly golden and firm to the touch, about 40 minutes. Slip the parchment paper with the shortbread onto a hard surface. Immediately sprinkle the shortbread heavily with the baker's sugar. While still warm, use the ruler and a paring knife, pointed straight down, to cut the shortbread into 1- or 1 ½-inch pieces. Let cool completely. The shortbread tastes best at room temperature.